espanso: remove background process type on Darwin

This leads to a considerably slower expansion time for snippets, which
severely degrades the user experience.

Also, the launchd plist from the source omits this key:

    8daadcc949/espanso/src/res/macos/com.federicoterzi.espanso.plist

PR #5411

modules/services/espanso.nix

@@ -138,7 +138,6 @@ in {
           Crashed = true;
           SuccessfulExit = false;
         };
-        ProcessType = "Background";
         RunAtLoad = true;
       };
     };

tests/modules/services/espanso-darwin/launchd.plist

@@ -16,8 +16,6 @@
 	</dict>
 	<key>Label</key>
 	<string>org.nix-community.home.espanso</string>
-	<key>ProcessType</key>
-	<string>Background</string>
 	<key>ProgramArguments</key>
 	<array>
 		<string>@espanso@/bin/espanso</string>