chore: remove submodule accidentally added to repo

[pbrissaud]

What this PR does / why we need it:
Remove submodule file

Which issue(s) this PR fixes:
Fixes #15100

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.

[pharaujo]

@cyriltovena can this fix be backported to the 3.3.x release branch? 3.3.2 still has the ghost submodule reference which breaks submodule checkout:

$ git co v3.3.2
(...)
HEAD is now at 23b5fc2c9 fix: fix the docker driver build (backport release-3.3.x) (#15478)

$ git submodule update --recursive --init --depth=1
fatal: No url found for submodule path '_shared-workflows-dockerhub-login' in .gitmodules

[loki-gh-app]

The backport to release-3.3.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-15103-to-release-3.3.x origin/release-3.3.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 38fcef3f1a27305ad502927b9f7d4476b118488f

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-15103-to-release-3.3.x
# Create the PR body template
PR_BODY=$(gh pr view 15103 --json body --template 'Backport 38fcef3f1a27305ad502927b9f7d4476b118488f from #15103{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: remove submodule accidentally added to repo (backport release-3.3.x)' --body-file - --label 'size/XS' --label 'type/bug' --label 'backport' --base release-3.3.x --milestone release-3.3.x --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-15103-to-release-3.3.x

# Create a pull request where the `base` branch is `release-3.3.x` and the `compare`/`head` branch is `backport-15103-to-release-3.3.x`.

# Remove the local backport branch
git switch main
git branch -D backport-15103-to-release-3.3.x

[trevorwhitney]

Backport PR: #15775