Form-associated custom elements: Remove upstreamed tests.

Tests in custom-elements/tentative were upstreamed to WPT by
https://github.com/web-platform-tests/wpt/pull/15740 .

Bug: 905922
Change-Id: I3e0b24486562b69e6fa212fd93927d3811d13967
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619590
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661298}
7 files changed