Refactor: crmd: use new attrd op to clear fail counts

Previously, crmd's CRM_OP_CLEAR_FAILCOUNT handler cleared the fail-count and
last-failure attributes by name. Now, it uses attrd's ATTRD_OP_CLEAR_FAILURE
operation. This simplifies the code and will make future changes in
failure handling easier.

Since only the DC schedules clear ops, and we are bumping the CRM_FEATURE_SET
with this release, all nodes are guaranteed to support the new op, so
there is no problem with backward compatibility in mixed-version clusters.