Revert "[iOS] Use a storyboard for the launch screen"

This reverts commit 4fda29d85e405f179875056559ca0c9a2b643ca4.

Reason for revert: The CL introduces consistently failing tests. Probably more cleanup is required.

Example failures:

https://ci.chromium.org/ui/p/chromium/builders/ci/ios-simulator-noncq/63791/overview

Bug: 433485734
Original change's description:
> [iOS] Use a storyboard for the launch screen
>
> This CL updates how the launch screen is constructed, from a xib to a
> storyboard.
>
> It also simplifies the constraints a bit, removing unnecessary ones and
> ensuring that the launch screen view controller has exactly the same.
> This fixes an issue with logo size on iPhone landscape.
>
> Before:
> https://screencast.googleplex.com/cast/NTEzMzAxOTcwOTk2NDI4OHxiYTEwODk1Ni00YQ
> https://screencast.googleplex.com/cast/NTM4MDY4NTAzOTY2NTE1Mnw4MmQwODFjYi1mZA
> https://screencast.googleplex.com/cast/NjI1ODkxOTYxNjgwNjkxMnxhNjhjMzkzZS0wOA
>
> After:
> https://screencast.googleplex.com/cast/NjAwNjY5OTg3Njk0MTgyNHxmNzlkMzFiYi0wNQ
> https://screencast.googleplex.com/cast/NTc5MjUwMzY4Mzg3NDgxNnxjODE4OGE0NS05OA
> https://screencast.googleplex.com/cast/NjYyMjg1ODgzODAxNjAwMHw4ZGJkMzlhMi05Mw
>
> Bug: 433485734
> Change-Id: I0a7698b29ceda25a4feb59fa640068f167336ba2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6777585
> Auto-Submit: Gauthier Ambard <gambard@chromium.org>
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Commit-Queue: Mark Cogan <marq@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1490848}

Bug: 433485734
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I1a8da8a28058e1a8ab94fc2aef81ec5de333ec7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6780369
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Auto-Submit: Liviu Tinta <liviutinta@chromium.org>
Owners-Override: Liviu Tinta <liviutinta@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1491087}