[Logs Shared] remove observability dependency (elastic#162733)

After the obs ai assistant [moved to its own
plugin](elastic#162243) outside the
observability plugin, logs_shared shouldn't need to depend on
observability

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/logs_shared/kibana.jsonc

@@ -9,7 +9,6 @@
     "browser": true,
     "configPath": ["xpack", "logs_shared"],
     "requiredPlugins": ["data", "dataViews", "usageCollection", "observabilityShared", "observabilityAIAssistant"],
-    "optionalPlugins": ["observability"],
     "requiredBundles": [
       "kibanaUtils",
       "kibanaReact",