vp10: remove double MV value check.

vp10: remove double MV value check.

This has virtually no effect on coding efficiency, but it is more
logical from a theoretical perspective (since it makes no sense to
me that you would exclude a MV from a list just because it's sign-
inversed value is identical to a value already in a list), and it
also makes the code simpler (it removes a duplicate value check in
cases where signbias is equal between the two MVs being compared).

See issue 662.

Change-Id: I23e607c6de150b9f11d1372fb2868b813c322d37