Remove problems.md and revert isssue template

Author: Mike Goelzer

.github/ISSUE_TEMPLATE/project-proposal.md

@@ -16,6 +16,5 @@ detail and PR discussion(s).
 Project name: [insert name]
 Proposal: [link to markdown file in this repo]
 Initial PR: [reference PR number with the first proposal draft]
-Problem Addressed: [reference a problem from `/problems.md` in this repo]
 
 This issue is for project management of a proposal, deciding, scheduling etc. Discussion about the relative merits of the proposal is welcome here, but discussion about the content of the proposal is better in a PR to the proposal document.