[iOS] Fix AccessibilityLabel of the NewTab button in toolbar

The accessibility label was overridden right after it was set.
This CL removes the override.

Bug: 987234
Change-Id: If30c3b9e2cbaf46b6fb607d69b917e5b8bf60f21
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715291
Auto-Submit: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680423}