SystemTools: Simplify FileIsExecutable on Windows

Revert commit 41700ca4 (SystemTools: Fix FileIsExecutable on Windows,
2020-04-09).  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 commit 29f31d17 (SystemTools: Add Windows app
exec alias support to FileExists(), 2021-10-22), the implementation
using `FileExists` will find it anyway.  Client (e.g. CMake) will need
another way to avoid using that python app link incorrectly.