Revert "Show test issues as annotations in the GitHub Actions job summary"

This reverts commit 3a5ff35928.

Annotations were not actually appearing.
<https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions>

contrib/checkdiff.bash

@@ -13,7 +13,7 @@ edited () {
 
 dependency () {
 	if edited "$1" && ! edited "$2"; then
-		echo >&2 "::warning '$1' was modified, but not '$2'! $3" | xargs
+		echo "'$1' was modified, but not '$2'! $3" | xargs
 	fi
 }