FileUtil: Remove redundant statement

[Dentomologist]

There are several problems with this statement:

• It tries to return [Directory containing Dolphin]/Dolphin.app/Contents/MacOS instead of the [Directory containing Dolphin]/Dolphin.app that is assumed by callers of the function.
• It uses std::string's find_last_of function, which returns the last instance of any character in the string rather than the last occurrence of the string as a whole.
• The haystack string is initialized with GetBundleDirectory() which returns [Directory containing Dolphin]/Dolphin.app, which would make the search fail if it were actually using a function that found the last instance of the string.

These errors coincidentally cancel each other out, but better to just remove the statement.