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/DirIterator.h

@@ -44,6 +44,4 @@ private:
     bool advance_next();
 };
 
-String find_executable_in_path(String filename);
-
 }