Clean up collector regex

What does this MR do and why?

1. Removes the logic where nil collector host was handled by replacing host from jitsu_host
2. Makes product_analytics_data_collector_host required for product_analytics_application_settings_defined?

Rationale

Application settings jitsu_host and collector_host should be stored separately. However, as a first iteration we used to store only jitsu_host and calculate the collector_host based on jitsu_host.

To make a proper fix, we

1. added a new product_analytics_data_collector_host to application_settings table to store the above-mentioned collector host.
2. added a new form field for collector host. If the value is empty, we still used jitsu host to calculate a data collector host
3. backfilled nil product_analytics_data_collector_host in db

Now that we have everything in place, we can remove the code where we handle nil collector host.

Note: This feature is behind a feature flag and thus a changelog isn't added.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #389490 (closed)