Fix for #23700 Non-deterministic static files test failures on Windows and Removing an unwanted check in collectstatic.py

Fix for #23700 Non-deterministic static files test failures on Windows:
PermissionError: [WinError 5] Access possibly raised when write
    permission on the parent directory is not there. In any case
    a permission check there would help. We might want to assert false
    when there is no permission to create the folder. Haven't added it
    in case it is handled elsewhere.

Removing an unwanted check in collectstatic.py:
Checking whether prefixed_path is not in self.copied_files seems
to be unnecessary since it is already being checked in the start
and there doesn't seem to be any place where self.copied_files is
getting modified after that.