Remove the with_connection_pool class method

This method was defined on Gitlab::Database and forwarded the call to an
instance of Gitlab::Database::Connection. Upon closer inspection it
turned out this method isn't used anywhere, nor is it documented. As
such we remove it in this commit, as there's no point in keeping it
around.

lib/gitlab/database.rb

@@ -138,10 +138,6 @@ module Gitlab
       "'f'"
     end
 
-    def self.with_connection_pool(...)
-      main.with_connection_pool(...)
-    end
-
     def self.sanitize_timestamp(timestamp)
       MAX_TIMESTAMP_VALUE > timestamp ? timestamp : MAX_TIMESTAMP_VALUE.dup
     end

lib/gitlab/database/connection.rb

@@ -166,16 +166,6 @@ module Gitlab
           .new.establish_connection(env_config)
       end
 
-      def with_connection_pool(pool_size)
-        pool = create_connection_pool(pool_size)
-
-        begin
-          yield(pool)
-        ensure
-          pool.disconnect!
-        end
-      end
-
       def cached_column_exists?(table_name, column_name)
         connection
           .schema_cache.columns_hash(table_name)

spec/lib/gitlab/database/connection_spec.rb

@@ -406,44 +406,6 @@ RSpec.describe Gitlab::Database::Connection do
     end
   end
 
-  describe '#with_connection_pool' do
-    it 'creates a new connection pool and disconnect it after used' do
-      closed_pool = nil
-
-      connection.with_connection_pool(1) do |pool|
-        pool.with_connection do |connection|
-          connection.execute('SELECT 1 AS value')
-        end
-
-        expect(pool).to be_connected
-
-        closed_pool = pool
-      end
-
-      expect(closed_pool).not_to be_connected
-    end
-
-    it 'disconnects the pool even an exception was raised' do
-      error = Class.new(RuntimeError)
-      closed_pool = nil
-
-      begin
-        connection.with_connection_pool(1) do |pool|
-          pool.with_connection do |connection|
-            connection.execute('SELECT 1 AS value')
-          end
-
-          closed_pool = pool
-
-          raise error, 'boom'
-        end
-      rescue error
-      end
-
-      expect(closed_pool).not_to be_connected
-    end
-  end
-
   describe '#cached_column_exists?' do
     it 'only retrieves data once' do
       expect(connection.scope.connection)