Big Switch: Don't clear hash before sync

This patch removes the step of clearing the consistency
hash from the DB before a topology sync. This will ensure
that inconsistency will be detected if the topology sync
fails.

This logic was originally there to make sure the hash header
was not present on the topology sync call to the backend.
However, the hash header is ignored by the backend in a sync
call so it wasn't necessary.

Conflicts:

	neutron/plugins/bigswitch/servermanager.py

Closes-Bug: #1379510
Change-Id: I2d58fa2aea3b692834d64192d06ace727c7df8a0
(cherry picked from commit 24e4110eb284078775496501ff81630eb1619c11)