Deprecate custom/default CtxLdr class name in TCF

Once the new bootstrap strategy for the TestContext framework (TCF) is
introduced in Spring Framework 4.1, a TestContextBootstrapper will
assume full responsibility for determining what ContextLoader to use as
the default. Consequently, the previous support for supplying the class
name for a custom, default ContextLoader will no longer be applicable.

This commit therefore officially deprecates support for custom, default
ContextLoader class names via the following mechanisms:

 - The TestContextManager(Class, String) constructor

 - The getDefaultContextLoaderClassName(Class) method in
   SpringJUnit4ClassRunner

Issue: SPR-11682