[Fleet] Remove code that handles security plugin being disabled

[joshdover]

Starting in 8.0, the Kibana security plugin can no longer be disabled. With #111681 merged, we can now:

1. Make security a required dependency in Fleet's kibana.json file
2. Remove all code in Fleet that handles the situation when the security plugin is disabled. For example:

   • kibana/x-pack/plugins/fleet/server/plugin.ts

     Line 285 in 504896d

     if (deps.security) {

   • kibana/x-pack/plugins/fleet/server/services/app_context.ts

     Lines 110 to 112 in 504896d

     	public hasSecurity() {
     	return !!this.securitySetup && !!this.securityStart;
     	}

Note that we still need to keep any checks that verify that security in Elasticsearch is enabled and configured.

This is not a required change at this time, just a tech debt cleanup.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)