[Metrics UI] Remove hard dependency on APM

[simianhacker]

Right now there is a hard dependency for APM to be enabled for the Metrics UI. This was originally introduced into the detail page metadata API for adding an APM section to that page. That PR was never merged because we decided to take a different approach.

We need to remove our hard dependency to the APM plugin. In 7.6 if you disable APM we throw a fatal error because the function provided by the APM plugin is undefined. In master (and anything with the new platform), disabling APM also disables Metrics and Logs UI because there is a hard dependency in the framework.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)