EX-3825 pcc: no error to detach missing file

EX-3825 pcc: no error to detach missing file

Do not print an error message if "lfs pcc detach" tries to detach
a file that is already removed from the cache.  This might happen
for a wide variety of reasons (external cache cleanup process, etc).

Test-Parameters: trivial testlist=sanity-pcc
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iad07b4bbab809bd356ed06c9f5cfcc35fa3ebbe5
Reviewed-on: https://review.whamcloud.com/45132
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng, Lei <flei@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>