Removed cgi/MultiMapping.py which is no longer in use.

Thanks to Cheer Xiao for reporting.
Analysis:
I've looked into this, seems MultiMapping.py was copied to
roundup/cgi/PageTemplates/MultiMapping.py in 2002 (!) and is dead since
then. It got some patches in 2004 however which maybe should have gone
into the other version.
The version in PageTemplates is used by TALES.py

Seems the changes in 2004 changed the following:
- add support for *setting* items
  Since the derived SafeMapping in TALES.py explicitly tries to prevent
  changes this is not needed.
- sort order of stores was reversed which leads to different
  implementations of push and pop (making them less efficient imho)
  but as far as I understand the code this only changes the internal
  representation
- new pop is non-failing (returns None if no stores)
  the implementation moves push and pop to _push and _pop and doesn't use
  these -- and templating code doesn't modify
- add has_key, keys, values, copy, items
  not used anywhere
- features a much less efficient implementation of len which uses items()
  to iterate instead of adding up the length of all stores.
- uses a "seen" mark maybe for future addition of a cache

The other MultiMapping has meanwhile grown another implementation of
items which seems much leaner.

Author: schlatterbeck