Drop vestigial test_backend setting (#611)

data-machine-code set test_backend=host-smoke, but its smoke scripts use the
smoke-*.php prefix convention, not the *-smoke.php suffix the WordPress test
dispatcher discovers. The host-smoke backend's `find -name '*-smoke.php'`
therefore matched zero files — the setting was a no-op. With the test_backend
toggle removed upstream (tests route by file type through WP Codebox), drop the
dead setting for hygiene.

Author: chubes4

homeboy.json

@@ -3,10 +3,7 @@
   "changelog_target": "docs/CHANGELOG.md",
   "extensions": {
     "wordpress": {
-      "release_latest_branch": "release-latest",
-      "settings": {
-        "test_backend": "host-smoke"
-      }
+      "release_latest_branch": "release-latest"
     }
   },
   "id": "data-machine-code",