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due to the old value no longer being referenced in internal gnutls calling of these two functions and also having been replaced by the latter flag.

Reference: #754 (closed)

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• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
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