Remove Product Analytics tracking destination

What does this MR do and why?

Cleaning up a POC by removing code introduced in !46482 (merged) and !39270 (merged).

Note that this is related to using the Product Analytics feature to track events that happen inside a GitLab instance. This MR does not change the Product Analytics feature itself.

Related to #342161 (closed)

How to set up and validate locally

1. Start Snowplow Micro and change the appropriate settings to point GDK to it. See instructions.
2. Start a Rails console.
3. Check the size of the product_analytics_events_experimental table:

   [1] pry(main)> ProductAnalyticsEvent.count
      (27140.6ms)  SELECT COUNT(*) FROM "product_analytics_events_experimental" /*application:console,db_config_name:main,line:/Users/ali/.rvm/gems/ruby-2.7.4/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'*/
   => 21202361

4. Fire a sample event:

   [2] pry(main)> Gitlab::Tracking.event('test-category', 'test-action')

5. Check that there is one good event reported at http://localhost:9090/micro/all.
6. Check that the size of product_analytics_events_experimental did not increase:

   [3] pry(main)> ProductAnalyticsEvent.count
      (27550.7ms)  SELECT COUNT(*) FROM "product_analytics_events_experimental" /*application:console,db_config_name:main,line:/Users/ali/.rvm/gems/ruby-2.7.4/gems/marginalia-1.10.0/lib/marginalia/comment.rb:25:in `block in construct_comment'*/
   => 21202361

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