Remove memory leak in HTTPSession.java

This is a nasty beast that has been bringing down our services using the NetCDF-Java OPeNDAP client.  Every HTTPSession instance created is added to a static ArrayList instance meaning the retained storage can not be GC'd.  A 17GB heap dump after an OOME showed 16.95 GB of references to Strings (and other objects) relating to HTTP Response headers from DAP calls.

From the code it looks like the functionality is designed to provide clean-up after tests.  I would recommend this session tracking and clean-up code be moved to the testing code base.

Can provide heap dump on request!