Revert "Use fixed flat maps for defining mojom::PrefPath mappings."

This reverts commit 4447fa4706df6c7ae30ab3ebfc215e466d5fd2ac.

Reason for revert: Suspected causing compile failure for many bots. Some example failures below

https://ci.chromium.org/ui/p/chromium/builders/ci/chromeos-amd64-generic-dbg/73821/overview
https://ci.chromium.org/ui/p/chromium/builders/ci/linux-chromeos-dbg/33442/overview

Original change's description:
> Use fixed flat maps for defining mojom::PrefPath mappings.
>
> This is more memory efficient and requires no heap allocations. However,
> since a constexpr FixedFlatMap requires constexpr input, various pref
> constants have been migrated to be defined using constexpr.
>
> Bug: 1459785
> Change-Id: I9c58fa904481e4e27abe174a15b8823fadbbde52
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4662520
> Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Reviewed-by: Anthony Cui <cuianthony@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1166798}

Bug: 1459785
Change-Id: I8df173bff3f48a2b1d55db27c368101d21c2362e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4671749
Reviewed-by: Patti Lor <patricialor@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Patti Lor <patricialor@chromium.org>
Auto-Submit: Patti Lor <patricialor@chromium.org>
Commit-Queue: Patti Lor <patricialor@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1167237}