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: I111d7dc1638fa8fc0a4c35d9cc4c0a1631300f89
Reviewed-on: http://gerrit.openafs.org/10433
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>