Get rid of last use of db_meth

Get rid of last use of db_meth

Nothing touches db_meth in ports. Thus only way a db_meth can be set is
now as a side effect X509V3_set_conf() in which case the db is an NCONF
database and the db_meth will be a thin wrapper of NCONF_get_section().
Make that explicit in the implementation, remove the guts of the unused
X509V3_get_string() and X509V3_string_free(), turn X509V3_section_free()
into a noop and replace several checks for ctx->db, ctx->db->meth, ...
with a simple ctx->db != NULL check.

ok beck jsing