Remove old-style processing-instruction based syntax from DOM Parts

This has been replaced (in a few long-past CLs) with the newer
brace-based syntax, e.g. `<div {{}}>`.

Bug: 40271855
Change-Id: I38fb6338170c721585ea71c635066f12d39f81e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5605653


Commit-Queue: David Baron <dbaron@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312573}