vbo: remove vbo_context::legacy_currval

vbo: remove vbo_context::legacy_currval

It's not nice when you have several variables pointing to the same array
and you wanna ask your editor "where is this used" and you only get an answer
for one of the four currval, legacy_currval, generic_currval, mat_currval,
which is quite useless, because you never see the whole picture.

Let's get rid of the additional pointers.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Mathias Fröhlich <Mathias.Froehlich@web.de>