Remove Thread History.txt.

This file lists the threads for BOSS for a given game. However, LOOT is not
BOSSv3 as originally imagined, so we won't need to refer to the BOSS threads
from LOOT. If the "Thread History.txt" files need to be inspected in the
future, they are still available both from this repository's history, but it
is also still included (and kept up-to-date) with the relevant BOSS repository
at https://github.com/boss-developers