remove obsolete STRUCT_MGVTBL_DEFINITION from mg.h

STRUCT_MGVTBL_DEFINITION was added in commit 0f4eea8fa1 in 5.004_67 as
part of ML thread "[PATCH 5.004_67] Fixes for broken MS compiler" and
stopped being used in commit acfe0abced in 5.7.2. STRUCT_MGVTBL_DEFINITION
was used to provide binary compatibility between C++ objs and C structs,
where extra  C++ data was stored with each function * raising the member
size to 16 bytes instead of 4 on a 32 bit build, also see
http://social.msdn.microsoft.com/Forums/en-US/vcgeneral/thread/2793a64f-ec09-495c-b995-4f5b98a26321

The define in struct mgvtbl is a left over that wasn't removed. Remove it
to improve code readability.