refactor: Use new GetConfigFilePath function

New function was introduced by willcl-ark <will@256k1.dev> in commit
56e370fbb9 from
https://github.com/bitcoin/bitcoin/pull/27073 and removes some duplicate code.

src/qt/guiutil.cpp

@@ -428,7 +428,7 @@ void openDebugLogfile()
 
 bool openBitcoinConf()
 {
-    fs::path pathConfig = GetConfigFile(gArgs.GetPathArg("-conf", BITCOIN_CONF_FILENAME));
+    fs::path pathConfig = gArgs.GetConfigFilePath();
 
     /* Create the file */
     std::ofstream configFile{pathConfig, std::ios_base::app};