Remove unnecessary Z-Index for #problemarea buttons/inputs (#813)

## Summary:
On our mobile app, we're noticing that hints are rendering above our keypad due to this style that sets the z-index to 3. I don't think we need z-index here at all as the #problemarea is only used on Mobile and in the content editor - neither of which need the index. 

Rather than using .perseus-interactive, I'm now simply setting the position directly. The relative positioning is still important to ensure that the "+" symbol appears beside the "Next Hint" button. 

Issue: LC-1445

## Test plan:
manual

Author: SonicScrewdriver

Reviewers: jeremywiebe, SonicScrewdriver

Required Reviewers:

Approved By: jeremywiebe

Checks: ⏭  Upload Coverage, ✅ Publish npm snapshot (ubuntu-latest, 16.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 16.x), ✅ Extract i18n strings (ubuntu-latest, 16.x), ❌ Cypress (ubuntu-latest, 16.x), ✅ Jest Coverage (ubuntu-latest, 16.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 16.x), ✅ Check builds for changes in size (ubuntu-latest, 16.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 16.x), ✅ gerald

Pull Request URL: #813

.changeset/green-ligers-roll.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Removing unused CSS from #problemarea.

packages/perseus/src/styles/perseus-renderer.less

@@ -96,13 +96,6 @@
 }
 
 .framework-perseus {
-    #problemarea {
-        input,
-        button {
-            .perseus-interactive;
-        }
-    }
-
     div.paragraph {
         font-family: "Lato", sans-serif;
         font-weight: 400;