staging: rtl8188eu: delete some dead code

Calling rtw_free_xmitframe() with a NULL "pxmitframe" parameter is a
no-op.  It appears that originally this code was part of a loop but it
was already dead code by the time that the driver was merged into the
kernel.

Fixes: 7bc88639 ("staging: r8188eu: Add files for new driver - part 17")
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YL5jFF9ldj0W9P1f@mwanda


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