Remove the Connection#pool_size method

This method is no longer in use.

lib/gitlab/database/connection.rb

@@ -31,10 +31,6 @@ module Gitlab
         scope.connection_db_config.configuration_hash.with_indifferent_access
       end
 
-      def pool_size
-        config[:pool] || Database.default_pool_size
-      end
-
       def username
         config[:username] || ENV['USER']
       end

spec/lib/gitlab/database/connection_spec.rb

@@ -23,24 +23,6 @@ RSpec.describe Gitlab::Database::Connection do
     end
   end
 
-  describe '#pool_size' do
-    context 'when no explicit size is configured' do
-      it 'returns the default pool size' do
-        expect(connection).to receive(:config).and_return({ pool: nil })
-
-        expect(connection.pool_size).to eq(Gitlab::Database.default_pool_size)
-      end
-    end
-
-    context 'when an explicit pool size is set' do
-      it 'returns the pool size' do
-        expect(connection).to receive(:config).and_return({ pool: 4 })
-
-        expect(connection.pool_size).to eq(4)
-      end
-    end
-  end
-
   describe '#username' do
     context 'when a username is set' do
       it 'returns the username' do