refactor: Remove IsArgSet guard when fallback value is provided

Checking for IsArgSet before calling GetArg while providing the args
default value as fallback is both confusing and fragile.

It is confusing, because the provided fallback is dead code. So it would
be better to just call GetArg without a fallback.

However, ignoring the fallback value is fragile, because it would not be
sanitized.

Fix all issues by sanitizing the fallback value.

src/bitcoin-cli.cpp

@@ -1059,7 +1059,7 @@ static void ParseGetInfoResult(UniValue& result)
     }
 #endif
 
-    if (gArgs.IsArgSet("-color")) {
+    {
         const std::string color{gArgs.GetArg("-color", DEFAULT_COLOR_SETTING)};
         if (color == "always") {
             should_colorize = true;