powerpc: Remove unused vgacon_remap_base & fix build break

The build is broken with CONFIG_PPC32=y, CONFIG_FB_VGA16=y and
CONFIG_VGA_CONSOLE=n.

The problem is that vgacon_remap_base is not defined. It's used in:

    #define VGA_MAP_MEM(x,s) (x + vgacon_remap_base)

Which is used in the vga16fb.c code.

Digging down it seems vgacon_remap_base is never initialised. It used to
be, back in arch/ppc (pplus.c and prep_setup.c), but none of that code
ever made it to arch/powerpc.

So given it's been unused for >6 years, remove it.

Whether vga16fb.c works on 32-bit is another question, but this patch
shouldn't affect it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>