Revert "Sframe encryptor implementation"

This reverts commit 4820cf11c56af592ea4ed02cbce7c0b1a7656fbe.

Reason for revert: Breaks downstream projects due to new third party dependency.

Original change's description:
> Sframe encryptor implementation
>
> This CL introduces the concrete Sframe encryptor that sits between the
> public key-management API and the media pipeline, implementing the
> sender-side wiring for Sframe end-to-end encryption.
>
> # High-level design
> The implementation follows a three-layer split that matches the existing
> WebRTC layering and respects the api/ visibility rules: 1. Public API
> layer (api/sframe/) - A narrow, stable interface that exposes only key
> management to applications: setting an encryption key on the sender
> side.
>
> 2. Internal media interface layer (modules/sframe/) - Extends the public
> key-management interface with the Encrypt and "max ciphertext size"
> entry points used by the media pipeline. This is the surface the media
> channel speaks to; application code never sees it.
>
> 3. Concrete impl layer (modules/sframe/) - Implements the media
> interface on top of the cisco/sframe library.
>
> Splitting key management from frame protection lets the public handle
> expose only the small surface applications need, while the media
> pipeline talks to a richer interface internally.
>
> Bug: webrtc:479862368
> Change-Id: Id93dba5874b6f5b1f32723755dd404af202f6ecf
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/481301
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#48065}

Bug: webrtc:479862368
Change-Id: Icef4fe1ce6e391b045f7b94087ee419b7899a96c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/485200
Commit-Queue: Johannes Kron <kron@webrtc.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48077}