Delete DB importers for Self monitoring

What does this MR do and why?

Deletes DB importer files related to Self monitoring removal.

Deletes create and delete service for Self monitoring project.

Removes related specs and routes for create/delete and status for SM project.

Should be merged in %16.0 and only after !115154 (merged) is merged.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

Related to #394987 (closed)

db/fixtures/development/98_gitlab_instance_administration_project.rb

-# frozen_string_literal: true
-
-response = Sidekiq::Worker.skipping_transaction_check do
-  result = ::Gitlab::DatabaseImporters::SelfMonitoring::Project::CreateService.new.execute
-
-  next result unless result[:status] == :success
-
-  AuthorizedProjectUpdate::ProjectRecalculateService.new(result[:project]).execute
-
-  result
-end
-
-if response[:status] == :success
-  puts "Successfully created self-monitoring project."
-else
-  puts "Could not create self-monitoring project due to error: '#{response[:message]}'"
-  puts "Check logs for more details."
-end