Remove Servlet 3.0 Support in AbstractRequestMatcherRegistry

[jzheaux]

Related to #6220

AbstractRequestMatcherRegistry checks first to make sure that the class javax.servlet.ServletRegistration is present.

Since Spring Framework 5.x increases it's Servlet Spec baseline to 3.1, this check is no longer necessary, and the class can assume its availability.

The task here, then, is to remove the check. Since isServlet30 is effectively always true now, the class should now always postProcess the ObjectPostProcessor instead of conditionally.

[farooqkhan003]

@jzheaux can I pick this?

[jzheaux]

It's yours, @farooqkhan003!

[jzheaux]

Fixed via 3b4ee1f