[iOS] Remove "rich" in-product-help bubble image functionality

After the multi-arm bubble type experiment conducted for Contextual
Panel, it has been concluded that rich in-product-help (IPH) bubbles
shouldn't have images in them, only title and subtext. Removing the
images functionality as this is Chrome for iOS UX-wide.

Bug: 394329576
Change-Id: Ie3339967e37fb36b226b554a40a0827e4686680f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6240093
Reviewed-by: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Nicolas MacBeth <nicolasmacbeth@google.com>
Cr-Commit-Position: refs/heads/main@{#1418626}