This change was made to prevent renaming files in the file selector from
deleting the file, see: #12255 & [0]. This check is no longer needed as
file selectors now checks the destination doesn't exist before renaming.
Generally, having low level file handling functions behave differently
between platforms is something to avoid with the potential of leading
to unexpected behavior.
Also unify WIN32/UNIX BLI_rename.
[0]: 41ad6f9d0a
8ab0196607