Remove special x509 test conversions

Following the commit from July 2 that removed netscape formated certs,
it is no longer necessary to have conversion tests for it.
Reviewed-by: NRich Salz <rsalz@openssl.org>

test/recipes/tconversion.pl

@@ -9,8 +9,9 @@ use lib 'testlib';
 use OpenSSL::Test qw/:DEFAULT top_file/;
 
 my %conversionforms = (
+    # Default conversion forms.  Other series may be added with
+    # specific test types as key.
     "*"		=> [ "d", "p" ],
-    x509	=> [ "d", "n", "p" ],
     );
 sub tconversion {
     my $testtype = shift;