build: Drop an unused Dockerfile (#36161)

* build: Drop an unused Dockerfile

We never cleaned up the docker file we were using for custom github
runners when we switched to standard runners.  This cleans that up and
removes another Docker mention that is out of date.

Co-authored-by: Kyle McCormick <[email protected]>

Author: feanil

scripts/ci-runner.Dockerfile

@@ -188,7 +188,7 @@ def compile_sass_dir(
         Eventually, we may eschew libsass-python altogether by switching to [email protected], a direct CLI for [email protected].
         (ref: https://github.com/sass/sassc). This would be nice because it would allow us to remove Python from the
         Sass build pipeline entirely. However, it would mean explicitly compiling & installing both libsass and SassC
-        within the edx-platform Dockerfile, which has its own drawbacks.
+        within the edx-platform build environment, which has its own drawbacks.
         """
         # Constants from libsass-python
         SASS_STYLE_NESTED = 0