Remove wpengine specific code

[markkelnar]

Remove the WP Engine specific code. This code exists in wpengine specific plugin, listening to the 'graphql_purge' action.

Dependent on wp-graphql change.

related to #246

[markkelnar]

Calling out the dependency on the wp-graphql or wpengine plugin.

The change in wp-graphql to use hostname as the URL cache header, means wpengine plugin purge hook that invalidates code on grapqhl_purge, will start working as the action that invalidates, instead of the smart-cache hook.

The only gap in functionality is if this smart-cache code is active (without the purge call to wpengine varnish with full https://hostname/path/) and older version of wp-graphql without the change above to hostname. In that situation graphql url header being used would include https://hostname/path but wpengine plugin would invalidate 'hostname' cache key. And cache would not invalidate.

Is it acceptable to document this and leave it up to the user to update versions of each to get/maintain cache invalidation behavior at wpengine?