Remove unnecessary friend class declarations.

Change-Id: Ida83136a083642bd6cd651c4cb506723e8ac9a41
Reviewed-on: https://swiftshader-review.googlesource.com/4570
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>