staging: r8188eu: remove empty function rtl8188eu_free_xmit_priv

staging: r8188eu: remove empty function rtl8188eu_free_xmit_priv

Remove empty function rtl8188eu_free_xmit_priv from hal/rtl8188eu_xmit.c
and its declaration from include/rtl8188e_xmit.h, as well as the one
place where it is stored in the free_xmit_priv function pointer of the
HalFunc struct inside padapter. This is safe, as the wrapper function
checks for NULL before calling this function via the function pointer.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210818234253.208271-5-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>