nmcli: do not save/restore terminal attrs

This is vestigal. It has been in place, because we'd be turning off echo ourselves when asking for password and needed to make sure we'd still terminal in original state upon unexpected termination.

This shouldn't be necessary since commit 9d95e1f1 ('clients/cli: use a nicer password prompt') we let readline take care of this and also clean up after itself in nmc_cleanup_readline().