ci: remove pull_request.types

cf. https://help.github.com/en/actions/reference/events-that-trigger-workflows#pull-request-event-pull_request
> By default, a workflow only runs when a pull_request's activity type is opened, synchronize, or reopened.

.github/workflows/test.yml

@@ -7,9 +7,6 @@ on:
     paths-ignore:
       - '**.md'
   pull_request:
-    types:
-      - opened
-      - synchronize
     paths-ignore:
       - '**.md'