Remove duplicate instruction from PR checklist (firebase#110)

2nd item already says:

```
- [ ] If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
```

.github/PULL_REQUEST_TEMPLATE.md

@@ -17,7 +17,6 @@ Before you create this PR confirm that it meets all requirements listed below by
 - [ ] I updated/added relevant documentation (doc comments with `///`).
 - [ ] The analyzer (`flutter analyze`) does not report any problems on my PR.
 - [ ] I read and followed the [Flutter Style Guide].
-- [ ] The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
 - [ ] I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
 - [ ] I updated CHANGELOG.md to add a description of the change.
 - [ ] I signed the [CLA].