staging: r8188eu: terminate_xmitthread_sema is not used

Remove terminate_xmitthread_sema from struct xmit_priv. This semaphore is
initialised but not used.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20221230180646.91008-16-martin@kaiser.cx


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