debian: remove `Recommends: chrony`

It isn't used anywhere - we only call `ntpdate` as part of
qubes.SyncNtpTime.

If user want to install it, he/she is free to do that.

(reported by @adrelanos)
Fixes qubesos/qubes-issues#1102

debian/control

@@ -38,7 +38,6 @@ Depends:
     ${shlibs:Depends},
     ${misc:Depends}
 Recommends:
-    chrony,
     cups,
     gnome-packagekit,
     gnome-terminal,