ci: fix pull_request trigger for coverage workflow

The coverage workflow is a part of the default testing, so there is no
sense to run this workflow when the 'full-ci' label is set. Moreover, it
cancelled the run of the default testing and started the run of the full
testing due to workflow `concurrency`.

By default, a workflow only runs when a `pull_request` event's activity
type is `opened`, `synchronize`, or `reopened`. That's why there is no
sense in the following construction:

  pull_request:
    types: [ opened, reopened, synchronize ]

So just removed the line related to event's activity type.

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci