Don't warn on unload in iframe

[wlach]

The parent frame takes care of warning in case of navigation events,
so this is unnecessary (it was also set to fire basically
unconditionally which meant that people were being warned unnecessarily
if e.g. in the report view)

No tests because there is no need to test something we're no longer doing. :)

Pull Request checklist

• [N/A] Documentation: If this feature has or requires documentation, the relevant docs have been updated.
• Changelog: This PR updates the changelog with any user-visible changes.
• [N/A] Tests: This PR includes thorough tests or an explanation of why it does not

[codecov]

Codecov Report

Merging #2884 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2884   +/-   ##
=======================================
  Coverage   72.53%   72.53%           
=======================================
  Files         247      247           
  Lines        6102     6102           
  Branches      994      994           
=======================================
  Hits         4426     4426           
  Misses       1657     1657           
  Partials       19       19