dpy_gfx_update_dirty expects DIRTY_MEMORY_VGA logging to be always on,
but that will not be the case soon.  Because it computes the memory
region on the fly for every update (with memory_region_find), it cannot
enable/disable logging by itself.

We could always treat updates as invalidations if dirty logging is
not enabled, assuming that the board will enable logging on the
RAM region that includes the framebuffer.

However, the function is unused, so just drop it.
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>