Remove unused interactive mode from support/install and support/geo-install

What does this MR do and why?

The interactive code path in support/install and support/geo-install (the if [ $# -eq 1 ] branch) prompted for the install directory, repo URL, and telemetry consent. As noted in !5958 (comment 3353688586), nobody uses it:

• CI never reaches it. Every CI call passes explicit positional args (bash -s - gdk <ref> true).
• The documented one-line install (curl ... | bash with no args) does not reach it either. It falls through to the else branch and uses defaults.
• The only way to trigger it was the undocumented curl ... | bash -s dummy.

This MR removes the dead branch and simplifies the argument handling in both scripts.

Changes

• support/install: remove the interactive branch. Positional args shift by one: $1 = install dir, $2 = branch, $3 = telemetry.
• support/geo-install: same removal. $1 = primary dir, $2 = secondary dir, $3 = branch, $4 = telemetry.
• Both scripts: the final call changes from _ "$0" "$@" to _ "$@". The old code consumed $1 as a dummy trigger so passing $0 was harmless. The new code reads $1 as the install directory, so $0 must not be forwarded.
• Both scripts: add a comment explaining how to customise the repo URL with GITLAB_REPO_URL.

All existing callers (support/ci/Dockerfile.verify, support/ci/Dockerfile.novel_*, .gitlab/ci/novel.gitlab-ci.yml) already pass args in the new order, so no CI changes are needed.

How to verify

The build-verify-image, novel:* and update:macos jobs exercise the install script directly in CI. Locally:

./support/install /tmp/gdk-test main false

should clone into /tmp/gdk-test on main with telemetry off.

MR acceptance checklist

• This MR has been reviewed against the MR acceptance checklist.
• This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.

Thanks to @ahmed-abdelrady-SE for the initial investigation into which callers reach the interactive path.

Closes #3314 (closed)