pubkey: remove deprecated OLD_PUBKEY_VERIFY_FLAG_TLS1_RSA

gnutls_certificate_verify_flags comparisons in lib/pubkey.c against deprecated OLD_PUBKEY_VERIFY_FLAG_TLS1_RSA define (previously GNUTLS_PUBKEY_VERIFY_FLAG_TLS1_RSA) break functions gnutls_pubkey_verify_data2 and gnutls_pubkey_verify_hash2 when GNUTLS_VERIFY_DISABLE_CA_SIGN gnutls_certificate_verify_flags is set.

The supplementary comparison against OLD_PUBKEY_VERIFY_FLAG_TLS1_RSA has been removed in favor of the exclusive comparison against GNUTLS_VERIFY_USE_TLS1_RSA gnutls_certificate_verify_flags due to the old value no longer being referenced in internal gnutls calling of these two functions and also having been replaced by the latter flag.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code