Remove dh512.pem

Reviewed-by: NRich Salz <rsalz@openssl.org>

apps/dh512.pem

------BEGIN DH PARAMETERS-----
-MEYCQQD1Kv884bEpQBgRjXyEpwpy1obEAxnIByl6ypUM2Zafq9AKUJsCRtMIPWak
-XUGfnHy9iUsiGSa6q6Jew1XpKgVfAgEC
------END DH PARAMETERS-----
-
-These are the 512 bit DH parameters from "Assigned Number for SKIP Protocols"
-(http://www.skip-vpn.org/spec/numbers.html).
-See there for how they were generated.
-Note that g is not a generator, but this is not a problem since p is a safe prime.

doc/ssl/SSL_CTX_set_tmp_dh_callback.pod

@@ -83,9 +83,8 @@ which use safe primes and were generated verifiably pseudo-randomly.
 These files can be converted into C code using the B<-C> option of the
 L<dhparam(1)|dhparam(1)> application. Generation of custom DH
 parameters during installation should still be preferred to stop an
-attacker from specializing on a commonly used group. Files dh1024.pem
-and dh512.pem contain old parameters that must not be used by
-applications.
+attacker from specializing on a commonly used group. File dh1024.pem
+contains old parameters that must not be used by applications.
 
 An application may either directly specify the DH parameters or
 can supply the DH parameters via a callback function.