tests: drop prereq `PREPARE_FOR_MAIN_BRANCH` where no longer needed

We introduced the `PREPARE_FOR_MAIN_BRANCH` prereq for the sole purpose
of allowing us to perform the non-trivial adjustments regarding the
`master` -> `main` rename before the automatable ones.

Now that the transition is almost complete, we can stop using it in most
instances. The only two exceptions are t5526 and t9902: at the time of
writing, there are other patches in flight that touch these test
scripts, therefore their transition to `main` is postponed to a later
date.

This patch is the result of this command:

	sed -i 's/PREPARE_FOR_MAIN_BRANCH[ ,]//' t/t[0-9]*.sh &&
	git checkout HEAD -- t/t5526\* t/t9902\*

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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