[ci] Don't add a testing target for libclc

According to llvm#111369 (comment)
there is no testing to be done here.

Adding "check-all" only risks duplicating tests
if other project specific "check-" targets are
also added.

.ci/generate-buildkite-pipeline-premerge

@@ -219,7 +219,7 @@ function check-targets() {
       echo "check-all"
     ;;
     libclc)
-      echo "check-all"
+      # Currently there is no testing for libclc.
     ;;
     *)
       echo "check-${project}"