Revert "bug #10908 [HttpFoundation] implement session locking for PDO (Tobion)"

This reverts commit 8c71454f47bbcdf82693a0501acac3e8fe6e08cf, reversing
changes made to 735e9a4768962b6d5098733c347cff0df6b9cd36.
Merge branch '2.3' into 2.4

* 2.3:
  Revert "bug #10908 [HttpFoundation] implement session locking for PDO (Tobion)"
  bumped Symfony version to 2.3.15
  updated VERSION for 2.3.14
  update CONTRIBUTORS for 2.3.14
  updated CHANGELOG for 2.3.14

Conflicts:
	src/Symfony/Component/HttpKernel/Kernel.php