libvpxenc: remove some unused ctrl id mappings

VP8E_UPD_ENTROPY, VP8E_UPD_REFERENCE, VP8E_USE_REFERENCE were removed
from libvpx and the remaining values were never used here

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Zern <jzern@google.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>