[Growth] Clean ProfilePickerTextVariations experiment.

As the experiment is stopped and will not be resumed, this CL removes
the `ProfilePickerTextVariations` feature flag and all associated code.
This includes the different title and subtitle strings, the logic to
select them based on the feature parameter, and the browser tests for
these variations.

Fixed: 510689686
Change-Id: I32a3f5801377acb817794ffd5a207a7fad01d6ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7827630
Reviewed-by: Ryan Sultanem <rsult@google.com>
Commit-Queue: Sasha Kunitskaya <saskekun@google.com>
Reviewed-by: Kacper Bajkiewicz <kbajkiewicz@google.com>
Cr-Commit-Position: refs/heads/main@{#1628477}
NOKEYCHECK=True
GitOrigin-RevId: 476aa93f3bafc131fc9810cb1b7ca5014b87cc72