Stop installing mysql and postgres

Zuul doesn't have a database, so there's no need to install one, let
alone two, in its bindep.txt file. Remove them.

Change-Id: Ib1ac968d06d059d6184a8fec031cf198d0439596
diff --git a/bindep.txt b/bindep.txt
index 1f08a58..a2cc02e 100644
--- a/bindep.txt
+++ b/bindep.txt
@@ -1,5 +1 @@
-mysql-client [test]
-mysql-server [test]
-postgresql [test]
-postgresql-client [test]
 libjpeg-dev [test]