nixosTests.prosody: remove explicit timeout_callback error path

Slixmpp is yielding a IqTimeout error when timeout is reached. This
exception should be catched by the catchall exept clause. Removing the
useless timeout_callback function.

Kudos to
https://lab.louiz.org/poezio/slixmpp/-/merge_requests/198#note_16939
for that one.

nixos/tests/xmpp/xmpp-sendmessage.nix

@@ -51,11 +51,8 @@ class CthonTest(ClientXMPP):
         log.info('Message sent')
 
         # Test http upload (XEP_0363)
-        def timeout_callback(arg):
-            log.error("ERROR: Cannot upload file. XEP_0363 seems broken")
-            sys.exit(1)
         try:
-            url = await self['xep_0363'].upload_file("${dummyFile}",timeout=10, timeout_callback=timeout_callback)
+            url = await self['xep_0363'].upload_file("${dummyFile}",timeout=10)
         except:
             log.error("ERROR: Cannot run upload command. XEP_0363 seems broken")
             sys.exit(1)