Userland: Consolidate most PATH resolving into a single implementation

We previously had at least three different implementations for resolving
executables in the PATH, all of which had slightly different
characteristics.

Merge those into a single implementation to keep the behaviour
consistent, and maybe to make that implementation more configurable in
the future.

Userland/Libraries/LibCore/System.h

@@ -161,7 +161,6 @@ ErrorOr<Array<int, 2>> pipe2(int flags);
 #ifndef AK_OS_ANDROID
 ErrorOr<void> adjtime(const struct timeval* delta, struct timeval* old_delta);
 #endif
-ErrorOr<String> find_file_in_path(StringView filename);
 enum class SearchInPath {
     No,
     Yes,