submodule--helper: drop "update --prefix <pfx>" for "-C <pfx> update"

Since 29a5e9e1ff (submodule--helper update-clone: learn --init,
2022-03-04) we've been passing "-C <prefix>" from "git-submodule.sh"
whenever we pass "--prefix <prefix>", so the latter is redundant to
the former. Let's drop the "--prefix" option.

Suggested-by: Glen Choo <chooglen@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

git-submodule.sh

@@ -343,7 +343,6 @@ cmd_update()
 		${recursive:+--recursive} \
 		${init:+--init} \
 		${nofetch:+--no-fetch} \
-		${wt_prefix:+--prefix "$wt_prefix"} \
 		${rebase:+--rebase} \
 		${merge:+--merge} \
 		${checkout:+--checkout} \