[lens] Remove Zero State CSB and Straight to SRP Lens features

This commit removes the Zero State CSB (Contextual Search Box) and
Straight to SRP (Search Results Page) features from the Lens overlay.
These features are no longer required, and their removal addresses
technical debt by cleaning up obsolete experimental code.

Specific modifications include:
* Removing the `kLensSearchZeroStateCsb` and
`kLensOverlayStraightToSrp` feature flags.
* Deleting the associated routing and initialization logic within
`LensSearchController`.
* Cleaning up field trial configurations and related about_flags
entries.

Bug: b:449767015
Change-Id: I0fbf1d9615b1a9078712f0036f133f372e8d47b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7809055
Auto-Submit: Juan Mojica <juanmojica@google.com>
Commit-Queue: Juan Mojica <juanmojica@google.com>
Reviewed-by: Bryan Nguyen <nguyenbryan@google.com>
Reviewed-by: Ananya Seelam <ananyaseelam@google.com>
Cr-Commit-Position: refs/heads/main@{#1627859}
NOKEYCHECK=True
GitOrigin-RevId: 2814891acc2463ed891d5249cbce14a7589aebab