[coreboot] HAVE_MP_TABLE fixup

[Myles Watson]

I wanted to compile without an mptable for debugging, but it wasn't as
easy as I'd hoped.  This makes it easier.

Since the asus/m2v_mx_se doesn't need mptables, remove mptable.c now
that it compiles without it.

Build tested.

Signed-off-by: Myles Watson

Thanks,
Myles