staging: r8723au: Remove unused semaphores

The semaphores xmit_sema, terminate_xmitthread_sema, tx_retevt and
io_req have no users, hence remove all references to them.
Semaphores are going away in the future.

Signed-off-by: Binoy Jayan <binoy.jayan@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>