Fix EZP-21510: Performance in ClearViewCacheSubtree content action.

JIRA: https://jira.ez.no/browse/EZP-21510

in content/action, Instead of clearing cache for objects one by one, a list of node ids should be used for performance, and so that the CacheThreshold setting is met.

This removes the existing code in favor of `eZContentObjectTreeNode::clearViewCacheForSubtree`, which already does exactly that.
Ref: https://github.com/ezsystems/ezpublish-legacy/blob/master/kernel/classes/ezcontentobjecttreenode.php#L6134