remove back-*/external.h

servers/slapd/back-bdb/external.h

servers/slapd/back-bdb/init.c

servers/slapd/back-bdb/proto-bdb.h

servers/slapd/back-dnssrv/external.h

servers/slapd/back-dnssrv/init.c

servers/slapd/back-dnssrv/proto-dnssrv.h

servers/slapd/back-hdb/external.h

servers/slapd/back-ldap/external.h

servers/slapd/back-ldap/init.c

servers/slapd/back-ldap/proto-ldap.h

servers/slapd/back-ldbm/external.h

servers/slapd/back-ldbm/init.c

servers/slapd/back-ldbm/proto-back-ldbm.h

servers/slapd/back-meta/external.h

servers/slapd/back-meta/init.c

servers/slapd/back-meta/proto-meta.h

servers/slapd/back-monitor/external.h

servers/slapd/back-monitor/init.c

servers/slapd/back-monitor/proto-back-monitor.h

servers/slapd/back-null/external.h