[Trusted Types] Remove TT checks for child_node.idl.

Trusted Types initially tried to check all text insertions into
HTMLScriptElements. This turned out difficult and instead the
HTMLScriptElement itself got the script_text_internal_slot mechanism
that "caches" the TT-checked script text and then compares this cached
copy to the actual test when preparing the script. This mechanism is more
robust.

The old TT implementation partially kept the original checks for scripts
around. The updated spec removes all of them, and relies exclusively on
the script_text_internal_slot. Thus, we simply remove the now-superfluous
checks under the TrustedTypesHTML flag.

This fixes legacy-trusted-scripts and Node-multiple-arguments-tt-enforced WPT tests.

Spec discussion:
- https://github.com/web-platform-tests/wpt/pull/49920
- https://github.com/w3c/trusted-types/issues/494#issuecomment-2572883416

Bug: 330516530
Change-Id: I84faefb050accb816031fe03f23ca08059fb8ee9
Fixed: 355484097, 388117624
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6377483
Reviewed-by: Yifan Luo <lyf@chromium.org>
Commit-Queue: Daniel Vogelheim <vogelheim@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440278}