style: remove unneeded lint checkbox in pull_request_template.md

[samlaf]

Why are these changes needed?

Not sure why we have this checkbox since we have a CI action that lints. We should instead just make it impossible to merge a PR where the lint workflow is failing (I can't change these settings because not admin on this repo).

Checks

• I've made sure the lint is passing in this PR.
• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(

[dmanc]

We could also shift this further left by using pre-commit hooks

• https://github.com/golangci/golangci-lint/blob/master/.pre-commit-hooks.yaml
• https://pre-commit.com/

[jianoaix]

It's there maybe due to that we were in a unhealthy state long time ago. But sounds fine to make lint non-optional and auto enforced (hence remove it from check box).