Revert "CI: build build-tools image" (#6156)

It turns out the issue with skipped jobs is not so trivial (because
Github checks jobs transitively), a possible workaround with `if:
always() && contains(fromJSON('["success", "skipped"]'),
needs.build-buildtools-image.result)` will tangle the workflow really
bad. We'll need to come up with a better solution.

To unblock the main I'm going to revert
https://github.com/neondatabase/neon/pull/6082.

CONTRIBUTING.md

@@ -70,17 +70,3 @@ We're using the following approach to make it work:
 - The label gets removed automatically, so to run CI again with new changes, the label should be added again (after the review)
 
 For details see [`approved-for-ci-run.yml`](.github/workflows/approved-for-ci-run.yml)
-
-## How do I add the "pinned" tag to an buildtools image?
-We use the `pinned` tag for `Dockerfile.buildtools` build images in our CI/CD setup, currently adding the `pinned` tag is a manual operation.
-
-You can call it from GitHub UI: https://github.com/neondatabase/neon/actions/workflows/update_build_tools_image.yml,
-or using GitHub CLI:
-
-```bash
-gh workflow -R neondatabase/neon run update_build_tools_image.yml \
-            -f from-tag=6254913013 \
-            -f to-tag=pinned \
-
-# Default `-f to-tag` is `pinned`, so the parameter can be omitted.
-```