Delete old GitHub Actions workflow file

As the hottest of the press features of GitHub Actions
requires using YAML rather than HCL.

.github/main.workflow

@@ -1,15 +0,0 @@
-workflow "Verify changes on push" {
-  resolves = ["Run unit tests"]
-  on = "push"
-}
-
-action "Install dependencies" {
-  uses = "actions/npm@59b64a598378f31e49cb76f27d6f3312b582f680"
-  args = "install"
-}
-
-action "Run unit tests" {
-  uses = "actions/npm@59b64a598378f31e49cb76f27d6f3312b582f680"
-  needs = "Install dependencies"
-  args = "test"
-}