staging: rtl8188eu: remove rtw_mp_phy_regdef.h

The header rtw_mp_phy_regdef.h is not used anywhere.
'git grep rtw_mp_phy_regdef.h' returns nothing, remove the file.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>