Revert "[Fast Pair] Add/remove Subsequent devices in quick_pair_mediator"

This reverts commit 0fbadf67244ec498b4d0c643faa8fc65bb69ce4b.

Reason for revert: Flag is expired and will not be landing

Original change's description:
> [Fast Pair] Add/remove Subsequent devices in quick_pair_mediator
>
> Adding the Add/Remove/Clear functions to the mediator to keep the Fast Pairable Devices list updated and notify observers.
>
> TESTED: Checked that the list was being updated appropriately by each function by deploying to DUT and using logs.
>
> Bug: b/290067219
> Change-Id: Ic34d1702e47d1ddcbe2ad34c8db51e5f44ffbd67
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4712728
> Reviewed-by: Daniel Classon <dclasson@google.com>
> Commit-Queue: Paola Lacouture <paolalac@google.com>
> Cr-Commit-Position: refs/heads/main@{#1179219}

Bug: b/378568944
Change-Id: Icc0be00a14d0793ff6d6105ff0b921a53c19a92f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6305648
Commit-Queue: Daniel Classon <dclasson@google.com>
Reviewed-by: Alex Kingsborough <akingsb@google.com>
Cr-Commit-Position: refs/heads/main@{#1427259}