cmSystemTools: Drop GetRealPathResolvingWindowsSubst

It was created by commit 83630d4918 (cmSystemTools: Revert GetRealPath
implementation on Windows, 2018-05-29, v3.11.3~3^2) to preserve support
on Windows for one call site.  Now that `GetRealPath` works again on
Windows, we can use that instead.

Issue: #18033
3 files changed