Remove reference to "if unsure if a bug" in issue template.

[oxinabox]

After the following exchange in the Stack Overflow comments:

• The iteration thing is intentional, the lastindex thing I do not think is. idx[end] makes sense as a thing to do. Would you care to raise an issue? – Lyndon White Apr 10 at 8:03
• @LyndonWhite I asked about this on Discourse but didn't get a response from anyone (and it has been over a week now)... – Colin T Bowers 15 hours ago
• Issues belong on github, not on discourse. Even potential issues that get rapidly closed as being features, or stemming from user error still belong on GitHub – Lyndon White 12 hours ago
• @LyndonWhite I actually did start at the GitHub issues page, but was put off by the default banner: If you have a question or are unsure if the behavior you're experiencing is a bug, please search or post to our Discourse site. I've opened an issue now lastindex not defined for CartesianIndex #31815 – Colin T Bowers 10 hours ago

I stand by my comment:
Bug reports (even "incorrect" ones), are much better handled here than on Discourse.

• People who watch the issues here are a different (overlapping) group to those who read discourse.
• In particular they are more likey to know if something was implemented with a particular behavour.
• People looking to see if something is a bug look in issues. And the closed issues act as a sign-post to tell them it isn't.

I think that exchange quoted above, shows this language is indeed directing people with legit issues away to discourse where they get silence for over a week. Vs raising an issue which got a response in <5 hours. By someone who is rarely on Discourse.

[JeffBezanson]

I agree; "incorrect" issues are quick to handle and are not a significant burden. I'd rather have an invalid issue filed than lose track of a real problem because it's on a discourse thread I didn't see.