staging: r8188eu: remove rtw_mfree2d() function

It is just a wrapper around kfree(), so remove it and just call kfree()
instead.

Link: https://lore.kernel.org/r/20210730092417.1014392-7-gregkh@linuxfoundation.org


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