t3412: further simplify setting of GIT_EDITOR

2182896 (t3412: clean up GIT_EDITOR usage, 2009-01-30) tried to clean up
the script's use of GIT_EDITOR, but it can further be simplified, because
that is how test-lib.sh sets things up already.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3412-rebase-root.sh

@@ -6,10 +6,6 @@ Tests if git rebase --root --onto <newparent> can rebase the root commit.
 '
 . ./test-lib.sh
 
-# we always run the interactive rebases unchanged, so just disable the editor
-GIT_EDITOR=:
-export GIT_EDITOR
-
 log_with_names () {
 	git rev-list --topo-order --parents --pretty="tformat:%s" HEAD |
 	git name-rev --stdin --name-only --refs=refs/heads/$1