lustre/fld: remove fld_thread_key

It is left over when porting Lustre tree patch in commit (e62e5d92) and should be
removed.

Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>