Revert "Merge pull request #19881 from sikachu/silence-mysql-errno-warning"

This reverts commit 55d9e494, reversing
changes made to 03e987cd.

Legacy mysql adapter is already removed in #22642.

activerecord/test/cases/disconnected_test.rb

@@ -23,9 +23,7 @@ def setup
       @connection.execute "SELECT count(*) from products"
       @connection.disconnect!
       assert_raises(ActiveRecord::ConnectionNotEstablished) do
-        silence_warnings do
-          @connection.execute "SELECT count(*) from products"
-        end
+        @connection.execute "SELECT count(*) from products"
       end
     end
   end