Remove obsolete function in COOP WPTs.

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}