Remove some obsolete/obscure internal define switches:

- FLAT_INC
- PKCS1_CHECK (the SSL_OP_PKCS1_CHECK options have been
  no-oped)
- PKCS_TESTVECT (debugging leftovers)
- SSL_AD_MISSING_SRP_USERNAME (unfinished feature)
- DTLS_AD_MISSING_HANDSHAKE_MESSAGE (unfinished feature)
- USE_OBJ_MAC (note this removes a define from the public header but
   very unlikely someone would be depending on it)
- SSL_FORBID_ENULL

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>