bgpd: Do not process workqueue upon instance delete

bgpd: Do not process workqueue upon instance delete

During instance cleanup, an earlier patch walked the workqueue in order
to process queued routes of the instance. However, since the workqueue
is not per instance, the code walks and immediately processes all routes
across all instances.

This may not be ideal in the presence of VRFs, when multiple instances
will be a fact. Revert that part of the change from earlier patch. This
needs to be revisited later for a better solution.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Updates: bb86c6017eccd03ea1554c7bdef5949cf676f208