Remove unused variables
Remove an impossible condition

uiIntraPeriod is unsigned, and thus can't ever be negative.

The subtraction on the line above can't ever make the value wrap
around either, since param_.uiIntraPeriod % uiGOPSize always is
smaller than or equal to param_.uiIntraPeriod. Thus, the check
never is necessary even if the value would be signed either.

This fixes warnings when building with clang.
Merge pull request #1471 from mstorsjo/fix-warnings

Remove unused variables and an impossible condition