ES|QL static metadata (#183890)

## Summary

Cleanups the implementation of the metadata logic. There is no reason at
this point (and maybe not even in the future) to have this metadata list
be dynamic. This PR is removing this logic and cleans it up

examples/esql_validation_example/public/code_snippet.tsx

@@ -35,11 +35,6 @@ function getCallbacksCode(callbacks: CodeSnippetProps['callbacks']) {
         ],`
             : ''
         }
-        ${
-          callbacks.metaFields
-            ? `getMetaFields: async () => ['_version', '_id', '_index', '_source'],`
-            : ''
-        }
         ${
           callbacks.policies
             ? `getPolicies: async () => [