Do not enque purges for ask based protocol

Do not enque purges for ask based protocol

With metadata disabled for ask based protocol, there is no need
to purge transaction on the backend and pollute the journal.

Tell based protocol has purges handled by the fronted so it
should be safe for us to remove them on the backend for ask
protocol.

JIRA: CONTROLLER-1879
Change-Id: Ia5c9470782bb4d59b78bbaa605ee5d36c7fb1644
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
(cherry picked from commit 4944f54d4e1fc24404d55e4ab74b6de212844dcd)
(cherry picked from commit 9ba29cd861d75d328aa67d53726250226dc8e438)