Remove no longer needed CSS rules

Since Nextcloud 22 the "#new-user" rules had no effect, as the DOM
structure changed to show a dialog rather than adding a row on top of
the list when adding new users.

Similarly, the z-index was no longer needed, as there will be no
"new-user" row that could overlap. Moreover, the z-index was set even
higher (100) in another rule still active.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>