Fix some TODO(3.0) occurrences in ssl/t1_lib.c

Fix some TODO(3.0) occurrences in ssl/t1_lib.c

One was related to probing for the combination of signature and hash
algorithm together. This is currently not easily possible. The TODO(3.0)
is converted to a normal comment and I've raised the problem as issue
number #14885 as something to resolve post 3.0.

The other TODO was a hard coded limit on the number of groups that could
be registered. This has been amended so that there is no limit.

Fixes #14333

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14886)