WebUI: Remove unnecessary "not ios" assertion in focus_without_ink.ts.

The assertion is no longer needed, as the minimum supported iOS version
has advanced to iOS 16 since this check was added. Moreover, this check
could lead to a Javascript runtime error like the following (revealed by
error reporting):

Uncaught Error: Assertion failed: pointerdown doesn't work on iOS
(chrome://new-tab-page)

When modifying the user agent via DevTools "Network predictions" panel
to return the Chrome iOS user agent, even if running on Desktop.

Fixed: 399203749
Change-Id: Id90b7170ede5da427e0d8125f4ad29eace5dd760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6427174
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1442257}