git-gui: drop no-op GITGUI_SCRIPT replacement

git-gui: drop no-op GITGUI_SCRIPT replacement

The value of the GITGUI_SCRIPT variable is only used in a single place
as part of an sed(1) script that massages the "git-gui.sh" script.
Interestingly, this specific replacement does seem to be a no-op: we
replace "^ argv0=$$0" with " argv=$(GITGUI_SCRIPT)", which has a value
of "$$0". The result would thus be completely unchanged.

Drop the replacement and its variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>