staging: r8188eu: Remove calls to _rtw_spinlock_free

This wrapper does nothing on Linux. When calls to it were removed, a number of
routines became empty and could also be removed.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>