scripted-diff: stop using the gArgs wrappers

They were temporary additions to ease the transition.

-BEGIN VERIFY SCRIPT-
find src/ -name "*.cpp" ! -wholename "src/util.h" ! -wholename "src/util.cpp" | xargs perl -i -pe 's/(?<!\.)(ParseParameters|ReadConfigFile|IsArgSet|(Soft|Force)?(Get|Set)(|Bool|)Arg(s)?)\(/gArgs.\1(/g'
-END VERIFY SCRIPT-

src/rpc/protocol.cpp

@@ -68,7 +68,7 @@ static const std::string COOKIEAUTH_FILE = ".cookie";
 
 fs::path GetAuthCookieFile()
 {
-    fs::path path(GetArg("-rpccookiefile", COOKIEAUTH_FILE));
+    fs::path path(gArgs.GetArg("-rpccookiefile", COOKIEAUTH_FILE));
     if (!path.is_complete()) path = GetDataDir() / path;
     return path;
 }