When ext-comp-refs is enabled, one-sided-compound is enabled by default,
which ensures the use of ext-comp-refs is an extension of
one-sided-compound. Both coding tools allow the use of same-sided
reference frame pairs for compound prediction.

Also, remove the dependency of ext-comp-refs on var-refs, i.e. these two
coding tools can be independently enabled. They can still work together
if both are enabled simultaneously.

Change-Id: I3134e7e2956dc35d557fe814f5d801d473683650