remove slightly varied background-color on main nav toggle

Fixes: CNVS-38402

test plan:
- navigate to the dashboard and using browser inspector
  ensure the background-color has been removed from
  `.ic-app-header__secondary-navigation` which is the
  container for the toggle button
- example: https://screencast.com/t/NFGT93CnUui

Change-Id: I430686a021489d19a1c529ed8f04255eef814fd5
Reviewed-on: https://gerrit.instructure.com/120854
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins

app/stylesheets/base/_ic_app_header.scss

@@ -64,7 +64,6 @@ $ic-tooltip-arrow-size: 0.375rem;
   bottom: 0;
   z-index: 1;
   width: 100%;
-  background-color: rgba($ic-brand-global-nav-bgd, 0.8);
 }
 
 .ic-app-header__menu-list {