staging: lustre: osc: Remove remains of osc_ast_guard

osc_ast_guard has been removed by the clio simplification.
Remove the last lock class definition.

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7148
Reviewed-on: http://review.whamcloud.com/16392


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>