GDBM is no longer relevant

doc/guide/preamble.sdf

@@ -133,7 +133,6 @@ Cyrus|http://cyrusimap.web.cmu.edu/generalinfo.html
 Cyrus SASL|http://asg.web.cmu.edu/sasl/sasl-library.html
 GNU|http://www.gnu.org/software/
 GnuTLS|http://www.gnu.org/software/gnutls/
-GDBM|http://www.gnu.org/software/gdbm/
 Heimdal|http://www.pdc.kth.se/heimdal/
 JLDAP|http://www.openldap.org/jldap/
 MIT Kerberos|http://web.mit.edu/kerberos/www/