staging: r8188eu: Remove empty rtw_mfree_xmit_priv_lock()

Remove rtw_mfree_xmit_priv_lock() because it is empty. Remove its only
call from within _rtw_free_xmit_priv().

Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20210819050808.28825-1-fmdefrancesco@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>