submodule update: remove "-v" option

submodule update: remove "-v" option

In e84c3cf3dc3 (git-submodule.sh: accept verbose flag in cmd_update to
be non-quiet, 2018-08-14) the "git submodule update" sub-command was
made to understand "-v", but the option was never documented.

The only in-tree user has been this test added in
3ad0401e9e6 (submodule update: silence underlying merge/rebase with
"--quiet", 2020-09-30), it wasn't per-se testing --quiet, but fixing a
bug in e84c3cf3dc3: It used to set "GIT_QUIET=0" instead of unsetting
it on "-v", and thus we'd end up passing "--quiet" to "git
submodule--helper" on "-v", since the "--quiet" option was passed
using the ${parameter:+word} construct.

Furthermore, even if someone had used the "-v" option they'd only be
getting the default output. Our default in both git-submodule.sh and
"git submodule--helper" has been to be "verbose", so the only way this
option could have matter is if it were used as e.g.:

    git submodule --quiet update -v [...]

I.e. to undo the effect of a previous "--quiet" on the command-line.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>