Remove custom-elements/state/idlharness.window.js

This test doesn't actually work since it references an idl file which
isn't defined. I manually verified that the CustomStateSet idl is
already tested by external/wpt/html/dom/idlharness.https.html so there
is no need for this separate test file anymore.

Change-Id: If4aca93159724cdb9e7d007933635a2d55d06f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5253030
Commit-Queue: Joey Arhar <[email protected]>
Reviewed-by: Stephen McGruer <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1255766}