t4200: drop irrelevant code

While setting up an unresolved merge for `git rerere`, we run `git
rev-parse` and `git fmt-merge-msg` to create a variable `$fifth` and a
commit-message file `msg`, which we then never actually use. This has
been like that since these tests were added in 672d1b789b ("rerere:
migrate to parse-options API", 2010-08-05). This does exercise `git
rev-parse` and `git fmt-merge-msg`, but doesn't contribute to testing
`git rerere`. Drop these lines.

Reported-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4200-rerere.sh

@@ -365,9 +365,6 @@ test_expect_success 'set up an unresolved merge' '
 	test_might_fail git config --unset rerere.autoupdate &&
 	git reset --hard &&
 	git checkout version2 &&
-	fifth=$(git rev-parse fifth) &&
-	echo "$fifth		branch fifth of ." |
-	git fmt-merge-msg >msg &&
 	ancestor=$(git merge-base version2 fifth) &&
 	test_must_fail git merge-recursive "$ancestor" -- HEAD fifth &&