Improve pull request template proposal

Looking through Kibana PRs many currently ignore the check boxes or just delete them. The goal here is to make the process easier, delete some unused options and align the checkboxes with what's most useful. We've had some discussions on the Uptime team about ways the PR template could be potentially improved. These changes are based on an extended discussion we had on the topic. We'd love to hear if other teams would be OK with these recommended changes.

The changes here are:

* Allow authors to just delete unnecessary items rather than strike through. Adding all the `~~` in markdown is painful
* Remove the unnecessary checkbox for release notes, the build bot catches this and blocks merges without that being set.
* Add a checkbox for testing in various browser sizes / mobile responsive devices
* Remove the IE 11 testing checkbox. This is too heavy for every PR, we skip this check on our PRs and think it's likely that other teams do as well.

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,16 +4,14 @@ Summarize your PR. If it involves visual changes include a screenshot or gif.
 
 ### Checklist
 
-Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.
+Delete any items that are not applicable to this PR.
 
-- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)
 - [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)
 - [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials
 - [ ] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios
 - [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)
+- [ ] This renders correctly on smaller devices using a responsive layout. (You can test this [in your browser](https://www.browserstack.com/guide/responsive-testing-on-local-server)
 
 ### For maintainers
 
 - [ ] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)
-- [ ] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)
-