[iOS] Clean up Text Fragments

WebKit supports the text fragments standard from iOS 16.1 onward 🎉, so
we don't need our own (redundant) implementation anymore. (In fact, it's
better not to have both operating on the same page, to avoid their
highlights interfering with ours, or vice-versa.)

Fixed: 381853455
Change-Id: Ib81831224b8ebfd57e58101fd51387604a657bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6060001
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1391763}