dirmngr: Remove superfluous attribute from the LDAP schema.

I accidently added a gpgSubCertID attribute not realizing that the
pgpSubKeyID already carries the long keyid. Remove that. Note that
the pgpkeyID has the short keyid and the long keyid has the name
pgpCertID.

• Signed-off-by: Werner Koch <wk@gnupg.org>