[SIEM] Remove unused references to actions

[rylnd]

As mentioned in #64842, siem does not explicitly rely on the actions plugin, but instead delegates through the triggers_actions_ui plugin.

With the rule actions work now stable, the references to e.g. actionsClient are unused with no plan for that to change.

Whether actions should be removed entirely from our kibana.json is a matter of opinion: it is a transient dependency of alerting (and of triggers_actions_ui).

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)