Fix for #23700 Non-deterministic static files test failures on Windows and #23701 Removing an unwanted check in collectstatic.py

#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.

#23700 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.

Apologies for the mixups. A little new to this.
This seems wrong because it would only mask an issue with STATIC_ROOT not existing.