Remove getInjectedVar from injected metadata

[eliperelman]

According to TODOs from #41990:

/**
   * exposed temporarily until https://github.com/elastic/kibana/issues/41990 done
   * use *only* to retrieve config values. There is no way to set injected values
   * in the new platform. Use the legacy platform API instead.
   * @deprecated
   * */
  injectedMetadata: {
    getInjectedVar: (name: string, defaultValue?: any) => unknown;
  };

The only thing publicly accessible from the injectedMetadata service is this API method, and now that exposeToBrowser exists, we should now be able to remove the injected metadata service entirely.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)