afs: afs_FlushDCache avoid dup cache drained check

afs_WakeCacheWaitersIfDrained is called as the last statement
of both afs_DiscardDCache and afs_FreeDCache.  There is no need
to perform the same check again before exiting afs_FlushDCache.

Change-Id: I11b4c42868ca67e4717fd431b1d33a85ce57bd0f
Reviewed-on: http://gerrit.openafs.org/10234
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>