Comment out a use of QMailThreadSortKey::deserialize.

The others are commented out, and this can't possibly work - because it is
defined as a template function, but implemented in the .cpp.

Done-with: John Brooks <john.brooks@dereferenced.net>
Change-Id: I2f44a0264058ba32a87ab3302a47628734f6a9aa
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>