t4014: clean up format.thread config after each test

t4014: clean up format.thread config after each test

The threading tests turn on format.thread, but never clean
up after themselves, meaning that later tests will also have
format.thread set.

This is more annoying than most leftover config, too,
because not only does it impact the results of other tests,
but it does so non-deterministically. Threading requires the
generation of message-ids, which incorporate the current
time, meaning a slow-running test script may generate
different results from run to run.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>