nixos/bitwarden_rs: fix startup on 32 thread machines

LimitNPROC=64 is too low for bitwarden_rs to start on a 32 thread machine.
Remove the limit.

This fixes:

```
bitwarden_rs[38701]: /--------------------------------------------------------------------\
bitwarden_rs[38701]: |                       Starting Bitwarden_RS                        |
bitwarden_rs[38701]: |--------------------------------------------------------------------|
bitwarden_rs[38701]: | This is an *unofficial* Bitwarden implementation, DO NOT use the   |
bitwarden_rs[38701]: | official channels to report bugs/features, regardless of client.   |
bitwarden_rs[38701]: | Send usage/configuration questions or feature requests to:         |
bitwarden_rs[38701]: |   https://bitwardenrs.discourse.group/                             |
bitwarden_rs[38701]: | Report suspected bugs/issues in the software itself at:            |
bitwarden_rs[38701]: |   https://github.com/dani-garcia/bitwarden_rs/issues/new           |
bitwarden_rs[38701]: \--------------------------------------------------------------------/
bitwarden_rs[38701]: [INFO] No .env file found.
bitwarden_rs[38701]: [2021-05-24 03:34:41.121][bitwarden_rs::api::core::sends][INFO] Initiating send deletion
bitwarden_rs[38701]: [2021-05-24 03:34:41.122][start][INFO] Rocket has launched from http://127.0.0.1:8222
bitwarden_rs[38701]: [2021-05-24 03:34:41.126][panic][ERROR] thread 'unnamed' panicked at 'failed to spawn thread: Os { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }': /build/rustc-1.52.1-src/library/std/src/thread/mod.rs:620
bitwarden_rs[38701]:    0: bitwarden_rs::init_logging::{{closure}}
bitwarden_rs[38701]:    1: std::panicking::rust_panic_with_hook
bitwarden_rs[38701]:    2: std::panicking::begin_panic_handler::{{closure}}
bitwarden_rs[38701]:    3: std::sys_common::backtrace::__rust_end_short_backtrace
bitwarden_rs[38701]:    4: rust_begin_unwind
bitwarden_rs[38701]:    5: core::panicking::panic_fmt
bitwarden_rs[38701]:    6: core::result::unwrap_failed
bitwarden_rs[38701]:    7: hyper::server::listener::spawn_with
bitwarden_rs[38701]:    8: hyper::server::listener::ListenerPool<A>::accept
bitwarden_rs[38701]:    9: std::sys_common::backtrace::__rust_begin_short_backtrace
bitwarden_rs[38701]:   10: core::ops::function::FnOnce::call_once{{vtable.shim}}
bitwarden_rs[38701]:   11: std::sys::unix:🧵:Thread:🆕:thread_start
bitwarden_rs[38701]:   12: start_thread
bitwarden_rs[38701]:   13: __GI___clone
bitwarden_rs[38701]: [2021-05-24 03:34:41.126][panic][ERROR] thread 'main' panicked at 'internal error: entered unreachable code: the call to `handle_threads` should block on success': /build/bitwarden_rs-1.20.0-vendor.tar.gz/rocket/src/rocket.rs:751
bitwarden_rs[38701]:    0: bitwarden_rs::init_logging::{{closure}}
bitwarden_rs[38701]:    1: std::panicking::rust_panic_with_hook
bitwarden_rs[38701]:    2: std::panicking::begin_panic_handler::{{closure}}
bitwarden_rs[38701]:    3: std::sys_common::backtrace::__rust_end_short_backtrace
bitwarden_rs[38701]:    4: rust_begin_unwind
bitwarden_rs[38701]:    5: core::panicking::panic_fmt
bitwarden_rs[38701]:    6: rocket:🚀:Rocket::launch
bitwarden_rs[38701]:    7: bitwarden_rs::main
bitwarden_rs[38701]:    8: std::sys_common::backtrace::__rust_begin_short_backtrace
bitwarden_rs[38701]:    9: std::rt::lang_start::{{closure}}
bitwarden_rs[38701]:   10: std::rt::lang_start_internal
bitwarden_rs[38701]:   11: main
```

nixos/modules/services/security/bitwarden_rs/default.nix

@@ -121,7 +121,6 @@ in {
         EnvironmentFile = [ configFile ] ++ optional (cfg.environmentFile != null) cfg.environmentFile;
         ExecStart = "${bitwarden_rs}/bin/bitwarden_rs";
         LimitNOFILE = "1048576";
-        LimitNPROC = "64";
         PrivateTmp = "true";
         PrivateDevices = "true";
         ProtectHome = "true";