Delete ISSUE_TEMPLATE.md

If we want a generic issue template like this I think we should add it to https://github.com/hypothesis/.github where it'll apply to all Hypothesis repos. But I'm not sure that we really want this issue template so perhaps we should just delete it?

.github/ISSUE_TEMPLATE.md

-<!--
-Thank you for reporting an issue with the Hypothesis client! This is our bug
-tracker. If you have any support questions please email support@hypothes.is
-instead.
-
-Please check that the issue you're reporting isn't already reported. You
-can use the GitHub issues search box to help with this.
--->
-
-### Steps to reproduce
-1.
-2.
-3.
-
-### Expected behaviour
-Tell us what should happen.
-
-### Actual behaviour
-Tell us what happens instead.
-
-### Browser/system information
-Tell us which browser, browser version, and operating system you were using when
-you encountered this issue.
-
-### Additional details
-If you have other information which might be helpful, please include it. If a
-screenshot will help to illustrate the issue you are reporting, please include
-one.