Noticed that some ALTREF_FRAMEs could have used compound modes for its
prediction but have been labeled as SINGLE_REFERENCE mode in the frame
header. This experiment is to remove the COMPOUND_REFERENCE mode from
the frame-level reference mode choices and only leave SINGLE_REFERENCE
and REFERENCE_MODE_SELECT the two choices in the frame header.

When turning on both ext-refs and ref-adapt, compared against ext-refs
itself, a small gain is achieved. In PSNR, the bitrate saving gains are
as follows:

lowres: Avg -0.120%; BDRate -0.128%
midres: Avg -0.155%; BDRate -0.128%

Change-Id: I2cfff8a6b7eaa65ef863dbdbc4dd086d3b586f8c