rebase: remove the rebase.useBuiltin setting

Remove the rebase.useBuiltin setting, which was added as an escape
hatch to disable the builtin version of rebase first released with Git
2.20.

See [1] for the initial implementation of rebase.useBuiltin, and [2]
and [3] for the documentation and corresponding
GIT_TEST_REBASE_USE_BUILTIN option.

Carrying the legacy version is a maintenance burden as seen in
7e097e27d3 ("legacy-rebase: backport -C<n> and --whitespace=<option>
checks", 2018-11-20) and 9aea5e9286 ("rebase: fix regression in
rebase.useBuiltin=false test mode", 2019-02-13). Since the built-in
version has been shown to be stable enough let's remove the legacy
version.

As noted in [3] having use_builtin_rebase() shell out to get its
config doesn't make any sense anymore, that was done for the purposes
of spawning the legacy rebase without having modified any global
state. Let's instead handle this case in rebase_config().

There's still a bunch of references to git-legacy-rebase in po/*.po,
but those will be dealt with in time by the i18n effort.

Even though this configuration variable only existed two releases
let's not entirely delete the entry from the docs, but note its
absence. Individual versions of git tend to be around for a while due
to distro packaging timelines, so e.g. if we're "lucky" a given
version like 2.21 might be installed on say OSX for half a decade.

That'll mean some people probably setting this in config, and then
when they later wonder if it's needed they can Google search the
config option name or check it in git-config. It also allows us to
refer to the docs from the warning for details.

1. 55071ea248 ("rebase: start implementing it as a builtin",
   2018-08-07)
2. d8d0a546f0 ("rebase doc: document rebase.useBuiltin", 2018-11-14)
3. 62c23938fa ("tests: add a special setup where rebase.useBuiltin is
   off", 2018-11-14)
3. https://public-inbox.org/git/nycvar.QRO.7.76.6.1903141544110.41@tvgsbejvaqbjf.bet/

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

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