Remove unused images

[aduth]

🛠 Summary of changes

Removes unused images from source code.

Discovered with a similar script as in 18F/identity-idp#11215:

for i in `ls images`
do
  if ! grep -q -R _articles -e $i
  then
    echo $i;
  fi
done

declare-incident-workflow-1.png: Added in #418, never used
declare-incident-workflow-2.png: Added in #418, never used
declare-incident-workflow-3.png: Added in #418, never used
idv-session-errors-failure.png: Added in #471, never used
idv-ssn-rate-limited.png: Added in #471, never used
statuspage-sample-message.png: Last reference removed in #389

📜 Testing Plan

Verify removed images aren't referenced anywhere.

[vrajmohan]

Looks good