rebase: remove transitory rebase.useBuiltin setting & env

rebase: remove transitory rebase.useBuiltin setting & env

Remove the rebase.useBuiltin setting and the now-obsolete
GIT_TEST_REBASE_USE_BUILTIN test flag.

This was left in place after my d03ebd411c6 (rebase: remove the
rebase.useBuiltin setting, 2019-03-18) to help anyone who'd used the
experimental flag and wanted to know that it was the default, or that
they should transition their test environment to use the builtin
rebase unconditionally.

It's been more than long enough for those users to get a headsup about
this. So remove all the scaffolding that was left inplace after
d03ebd411c6. I'm also removing the documentation entry, if anyone
still has this left in their configuration they can do some source
archaeology to figure out what it used to do, which makes more sense
than exposing every git user reading the documentation to this legacy
configuration switch.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>