Prune NIDs from TLS_SIGALGS.

There's no need to store more than the TLS values.

Change-Id: I1a93c7c6aa3254caf7cc09969da52713e6f8acf4
Reviewed-on: https://boringssl-review.googlesource.com/5348
Reviewed-by: Adam Langley <agl@google.com>