staging: r8188eu: remove rtw_zvmalloc preprocessor definition

Remove rtw_zvmalloc preprocessor definition from include/osdep_service.h
as it now has no callers.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210818234853.208448-6-phil@philpotter.co.uk


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