[CI] remove workflow dispatch branch (backport #1362)

[mergify]

This is an automatic backport of pull request #1362 done by Mergify.
Cherry-pick of b8c5418 has failed:

On branch mergify/bp/humble/pr-1362
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit b8c5418.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/humble-binary-build-main.yml
	modified:   .github/workflows/humble-binary-build-testing.yml
	modified:   .github/workflows/humble-semi-binary-build-main.yml
	modified:   .github/workflows/humble-semi-binary-build-testing.yml
	modified:   .github/workflows/humble-source-build.yml
	modified:   .github/workflows/rolling-binary-build-main.yml
	modified:   .github/workflows/rolling-semi-binary-build-testing.yml
	modified:   .github/workflows/rolling-source-build.yml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   .github/workflows/foxy-binary-build-main.yml
	both modified:   .github/workflows/foxy-semi-binary-build-main.yml
	both modified:   .github/workflows/galactic-binary-build-main.yml
	both modified:   .github/workflows/galactic-semi-binary-build-main.yml
	deleted by us:   .github/workflows/iron-source-build.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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