bisect; remove unused "git-bisect.sh" and ".gitignore" entry

Since 73fce29 (Turn `git bisect` into a full built-in, 2022-11-10)
we've used builtin/bisect.c instead of git-bisect.sh to implement the
"bisect" command.

Let's remove the unused leftover script, and the ".gitignore" entry for
the "git-bisect--helper", which also hasn't been built since
73fce29.

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: avar

.gitignore

@@ -20,7 +20,6 @@
 /git-archimport
 /git-archive
 /git-bisect
-/git-bisect--helper
 /git-blame
 /git-branch
 /git-bugreport