ofproto: Remove unused variable from expire

This resolves a regression introduced by "ofproto: Optimise OpenFlow flow
expiry".

Signed-off-by: Simon Horman <horms@verge.net.au>
netdev-linux: Cache result in netdev_linux_miimon_run()

In general the network devices don't change very often.
By caching the lookup made by netdev_linux_miimon_run()
calls to shash_add() and shash_destroy() destroy are avoided
reducing the use of malloc(), free() and hash_bytes().

Performance tests:

osrg/mpls-of12:
  + Using a match-all flow with actions=drop:
    - Rate at which frames were received increased from ~27.8kpps to ~32.2kpps.
  + Using a match-all flow with actions=normal:
    - Rate at which frames were received increased from ~23.4kpps to ~25.6kpps.

* Upstream:
  + Using a match-all flow with actions=drop:
    - Rate at which frames were received increased from ~23.3kpps to ~24.4kpps.

Signed-off-by: Simon Horman <horms@verge.net.au>