CLEANUP: removing unneeded servlet-api from codebase

I did a search for servlet-api in all poms in Repose.  No reference other than being called out in a dependency management section.

1. Removed the entire external/servlet-api directory
2. Removed the reference to this servlet-api from dependency mgmt section in core
3. Removed the module from external/pom.xml

Did a clean install, all unit tests passed.  Given no module is actually depending on this code or references "servlet-api" it is a safe assumption that this removal will not affect Repose functionally.  Meynard also took this one step further and ran his branch with this module removed through the pipeline, all functional tests passed.