Remove replica selection change FF

Drops :load_balancing_improved_caught_up_hosts_check.
The change should improve picking replica over primary even if not all
replicas are caught, which we required previously.

Changelog: performance

config/feature_flags/development/load_balancing_improved_caught_up_hosts_check.yml

----
-name: load_balancing_improved_caught_up_hosts_check
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/65248
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/334989
-milestone: '14.1'
-type: development
-group: group::memory
-default_enabled: false

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -147,14 +147,9 @@ module Gitlab
           raise 'Failed to determine the write location of the primary database'
         end
 
-        # FF disabled: Returns true if all hosts have caught up to the given transaction write location.
-        # FF enabled: Returns true if there was at least one host that has caught up with the given transaction and sets it.
+        # Returns true if there was at least one host that has caught up with the given transaction and sets it.
         def all_caught_up?(location)
-          if ::Feature.enabled?(:load_balancing_improved_caught_up_hosts_check)
           select_up_to_date_host(location)
-          else
-            @host_list.hosts.all? { |host| host.caught_up?(location) }
-          end
         end
 
         # Returns true if there was at least one host that has caught up with the given transaction.

spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -312,30 +312,6 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
 
       expect(lb.all_caught_up?('foo')).to eq(true)
     end
-
-    context 'when :load_balancing_improved_caught_up_hosts_check FF is disabled' do
-      before do
-        stub_feature_flags(load_balancing_improved_caught_up_hosts_check: false)
-      end
-
-      it 'returns true if all hosts caught up to the write location' do
-        expect(lb.host_list.hosts).to all(receive(:caught_up?).with('foo').and_return(true))
-
-        expect(lb.all_caught_up?('foo')).to eq(true)
-      end
-
-      it 'returns false if a host has not yet caught up' do
-        expect(lb.host_list.hosts[0]).to receive(:caught_up?)
-          .with('foo')
-          .and_return(true)
-
-        expect(lb.host_list.hosts[1]).to receive(:caught_up?)
-          .with('foo')
-          .and_return(false)
-
-        expect(lb.all_caught_up?('foo')).to eq(false)
-      end
-    end
   end
 
   describe '#retry_with_backoff' do