This experiment of ref-adapt removes the option of COMPOUND_REFERENCE
from the frame level reference_mode setup, and hence removes 1 bit
in the frame header syntax. It now allows only SINGLE_REFERENCE or
REFERENCE_MODE_SELECT to specify in the frame header.

No noticeable coding performance impact has been observed.

Change-Id: I1bed64f927f31d396e8174a4f0c9119242bf0b5c