submodule update: remove never-used expansion

The `prefix` variable is never set, so this conditional expansion is
always '' (which does nothing).

Signed-off-by: Glen Choo <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: chooglen

git-submodule.sh

@@ -238,7 +238,6 @@ cmd_update()
 {
 	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper update \
 		${wt_prefix:+--prefix "$wt_prefix"} \
-		${prefix:+--recursive-prefix "$prefix"} \
 		"$@"
 }