Allow file renaming when changing nothing but case.

As T80586: File Browser unable to rename with Upper/Lower case shows you cannot Rename in File Browser and change a file name from "test" to "Test", where the letters remain the same but only the case changes.

File Browser's renamebutton_cb that has an unnecessary check to ensure that the new name does not exist, even though the renaming function it calls, BLI_rename, handles this situation correctly.