Remove opaque_prf_input extension.

This code doesn't even get built unless you go out of your way to pass an
extension value at build time.

Change-Id: I92ffcdfb18505c96e5ef390c8954a54cee19967f
Reviewed-on: https://boringssl-review.googlesource.com/1063
Reviewed-by: Adam Langley <agl@google.com>