Remove Servlet Spec 2.5 Support for HttpSessionSecurityContextRepository

[jzheaux]

Related to #6220

HttpSessionSecurityContextRepository adds a request wrapper for supporting Servlet Spec 3's startAsync methods.

Because Spring Framework 5.x has moved its baseline to Servlet Spec 3.1, the code should always use this wrapper instead of checking the servlet spec version first.

Also, the Servlet 2.5 tests for this feature should be removed.

[dongmyo]

@jzheaux I'd like to take it

[jzheaux]

Thanks, @dongmyo! It's yours.