OFStateManager: remove support for expiring flows in Forwarding

Reviewer: @kenchiang

None of our switches do this, and a switch that did would have to reimplement
a bunch of expiration.c to ratelimit the flow-removed messages and send
flow-idle notifications when necessary.

Once we're fully transitioned to the new flowtable APIs, ind_core_flow_removed 
would be the only remaining user of the flow-id hashtable.