libopenh264enc: Remove a workaround for silencing warnings about unused...

libopenh264enc: Remove a workaround for silencing warnings about unused variables in the OpenH264 header

The 1.3 release branch of OpenH264 (as well as the master branch)
have been updated so that GCC no longer warns about this variable
as being unused.

Signed-off-by: Martin Storsjö <martin@martin.st>