builtin-commit.c: do not remove COMMIT_EDITMSG

builtin-commit.c: do not remove COMMIT_EDITMSG

git-commit tries to remove the file ./COMMIT_EDITMSG instead of
$GIT_DIR/COMMIT_EDITMSG after commit preparation (e.g. running
hooks, launching editor).
This behavior exists since f5bbc3225c4b07 "Port git commit to C".

Some test cases (e.g. t/t7502-commit.sh) rely on the existence of
$GIT_DIR/COMMIT_EDITMSG after committing and, I guess, many people
are used to it.  So it is best not to remove it.

This patch just removes the removal of COMMIT_EDITMSG.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>