Actually fix the memory leak by clearing up the data in m_items.

As it turned out, the ETM kept filling its QHash<Item::Id, Item>
without ever cleaning it up. This resulted in increasing memory
consumption of KMail until eventually all collections where fetched
at least once by the ETM. Note: It's not a true leak, i.e. the data
is still reachable and gets properly cleaned up on close. But during
runtime the ever increasing memory consumption is easily noticeable

Compared to before, KMail's memory consumption should now be rather
stable. See e.g. the memory profiles attached to the review request.

Note: The memory consumption does not go down properly, which might
be due to a bug in glibc, see:
http://sourceware.org/bugzilla/show_bug.cgi?id=14827

REVIEW: 106832
DIGEST: