Resolve "Drop application_settings.clickhouse_connection_string column"

What does this MR do and why?

Removes unused clickhouse_connection_string column.

How to set up and validate locally

Example below:

1. Run the migrations

   bundle exec rails db:migrate

2. In psql client check if column exists

   select * from application_settings limit 1;  
   select clickhouse_connection_string from application_settings limit 1;
   ERROR:  column "clickhouse_connection_string" does not exist
   LINE 1: select clickhouse_connection_string from application_setting...

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 #388803 (closed)