Remove the issue template (#355)

The issue template for this repository wasn't terribly helpful (the only
non-blank-issue option was for a release template), and the only
template is now out of date. This PR removes the template (but keeps the
PR template that reminds people to add a news entry).

Information on making releases is now on the wiki at
https://github.com/enthought/apptools/wiki/Releasing.