SystemTools: Revert "Add Windows app exec alias support to FileExists"

Revert !236 (merged) pending further investigation. It caused the same problem that we fixed previously in commit !175 (merged). The problem is that Windows has stubs like

%USERPROFILE%\AppData\Local\Microsoft\WindowsApps\python.exe

that exist on disk but do not actually work when executed.

See discussion in cmake/cmake#22743 (comment 1073421).