Remove obsolete function in COOP WPTs.

[chromium-wpt-export-bot]

percent_encode was used for a single test case in COOP parsing. That
test case does not use url_test anymore and can be removed.

See https://chromium-review.googlesource.com/c/chromium/src/+/3620319

Bug: 1318373
Change-Id: I2c0d5bab35841711357b0dcc43410fad9be863eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3627538
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Arthur Hemery <ahemery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999430}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[KyleJu]

The flakiness is existing. @past please admin merge

[WeizhongX]

WPT Command: python3 ./wpt run --channel=nightly --verify --verify-no-chaos-mode --verify-repeat-loop=0 --verify-repeat-restart=10 --github-checks-text-file=/home/test/artifacts/checkrun.md --affected base_head --log-mach-level=info --log-mach=- -y --no-pause --no-restart-on-unexpected --install-fonts --no-headless --verify-log-full --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/html/cross-origin-opener-policy/javascript-url.https.html?0-3	navigation: SAME_SITE; parentCOOP: unsafe-none; childCOOP: unsafe-none	PASS: 1/2, TIMEOUT: 1/2	Test timed out
/html/cross-origin-opener-policy/javascript-url.https.html?0-3	navigation: SAME_SITE; parentCOOP: unsafe-none; childCOOP: same-origin-allow-popups	PASS: 1/2, TIMEOUT: 1/2	Test timed out

These may be pre-existing or new flakes. Please try to reproduce (see the above WPT command, though some flags may not be needed when running locally) and determine if your change introduced the flake. If you are unable to reproduce the problem, please tag @web-platform-tests/wpt-core-team in a comment for help.

[hemeryar]

COOP tests are indeed known to flake on the WPT CI. This change is a no-op.