palemoon: Remove MOZ_PKG_SPECIAL, add AV1 configure flag

MOZ_PKG_SPECIAL: https://forum.palemoon.org/viewtopic.php?f=3&t=26796#p214729
"This is something we use for distinction for our infra. […] yeah just exclude it from your system packaging."

AV1: https://forum.palemoon.org/viewtopic.php?f=5&t=25843#p214767
"[It's] not on Linux and SunOS build instructions, only on Windows. But […] it is enabled for official Pale Moon"

pkgs/applications/networking/browsers/palemoon/default.nix

@@ -142,6 +142,8 @@ stdenv.mkDerivation rec {
     ac_add_options --enable-jemalloc
     ac_add_options --enable-strip
     ac_add_options --enable-devtools
+    # Missing from build instructions, https://forum.palemoon.org/viewtopic.php?f=5&t=25843#p214767
+    ac_add_options --enable-av1
 
     ac_add_options --disable-eme
     ac_add_options --disable-webrtc
@@ -159,8 +161,6 @@ stdenv.mkDerivation rec {
 
     ac_add_options --x-libraries=${lib.makeLibraryPath [ xorg.libX11 ]}
 
-    export MOZ_PKG_SPECIAL=gtk$_GTK_VERSION
-
     #
     # NixOS-specific adjustments
     #