Don't release primary connections in the DB LB

In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68042 we found
that releasing primary connections can result in test suite errors. This
can happen when tests reuse a connection also used by Puma, but release
it instead of letting the Puma thread release it. Reproducing this is
difficult, but it happens often enough to be a problem.

Since Rails already releases model connections for us, we don't actually
need to release primary connections in the first place. See the
following for more information:

- https://tenderlovemaking.com/2011/10/20/connection-management-in-activerecord.html
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68042#note_672850193

This commit changes the load balancer code to not release primary
connections, with the exception of one case where this _is_ needed.

Changelog: fixed

lib/gitlab/database/load_balancing/host.rb

@@ -163,8 +163,6 @@ module Gitlab
 
         def primary_write_location
           load_balancer.primary_write_location
-        ensure
-          load_balancer.release_primary_connection
         end
 
         def database_replica_location

lib/gitlab/database/load_balancing/primary_host.rb

@@ -16,7 +16,8 @@ module Gitlab
         end
 
         def release_connection
-          @load_balancer.release_primary_connection
+          # no-op as releasing primary connections isn't needed.
+          nil
         end
 
         def enable_query_cache!
@@ -51,8 +52,6 @@ module Gitlab
 
         def primary_write_location
           @load_balancer.primary_write_location
-        ensure
-          @load_balancer.release_primary_connection
         end
 
         def database_replica_location

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

@@ -20,10 +20,8 @@ RSpec.describe Gitlab::Database::LoadBalancing::PrimaryHost do
   end
 
   describe '#release_connection' do
-    it 'releases a connection from the pool' do
-      expect(load_balancer).to receive(:release_primary_connection)
-
-      host.release_connection
+    it 'does nothing' do
+      expect(host.release_connection).to be_nil
     end
   end