git-submodule.sh: rename unused variables

git-submodule.sh: rename unused variables

The 'mode' variable is not used in cmd_update for its original purpose,
rename it to 'dummy' as it only serves the purpose to abort quickly
documenting this knowledge.

The variable 'stage' is also not used any more in cmd_update, so remove it.

This went unnoticed as first each function used the commonly used
submodule listing, which was converted in 74703a1e4df (submodule: rewrite
`module_list` shell function in C, 2015-09-02). When cmd_update was
using its own function starting in 48308681b07 (git submodule update:
have a dedicated helper for cloning, 2016-02-29), its removal was missed.

A later patch in this series also touches the communication between
the submodule helper and git-submodule.sh, but let's have this as
a preparatory patch, as it eases the next patch, which stores the
raw data instead of the line printed for this communication.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>