Drop XPI support from Mozilla converters

We no longer use it and it hasn't been actively supported.

Part of work for bug 3024.
Drop storage.xpi module

No longer used.

Part of work for bug 3024.
Drop misc.zipfileext module

No longer used.

Part of work for bug 3024.
Drop obsolete guide for migrating to firefox

This is the last part to fix bug 3024.
Fix broken :ref: link