SystemTools: Simplify FileIsExecutable on Windows

Revert !179 (merged). The purpose of that change was to avoid treating the path %USERPROFILE%\AppData\Local\Microsoft\WindowsApps\python.exe as an executable because it might point at an application installer instead of a working Python. Since !236 (merged), the implementation using FileExists will find it anyway. Client (e.g. CMake) will need another way to avoid using that python app link incorrectly.