Revert "sdp: introduce MCD::AttributeLevel for session/media-level attrs"

This reverts commit 60094aab910f7fda6cf40f4334e7369ee4d81a23.

Reason for revert: Breaks downstream projects. The definitions in session_description.h must be kept in parallel until downstream projects are updated. 

Original change's description:
> sdp: introduce MCD::AttributeLevel for session/media-level attrs
>
> Generalizes the enum used to track whether an attribute is signalled at
> session level or media level. The previous enum was specific to
> extmap-allow-mixed (ExtmapAllowMixed { kNo, kSession, kMedia }); rename
> it to AttributeLevel { kNone, kSession, kMedia } so it can be reused.
>
> Migrates extmap-allow-mixed to the new type:
>   ExtmapAllowMixed              -> AttributeLevel
>   kNo                           -> kNone
>   set_extmap_allow_mixed_enum() -> set_extmap_allow_mixed_level()
>   extmap_allow_mixed_enum()     -> extmap_allow_mixed_level()
>
> No (intended) behavior change.
> A follow-up CL will use AttributeLevel for cryptex.
>
> Bug: webrtc:455813732
> Change-Id: If68ad7e32dbfea61f3e122b719488d18ceb88290
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470580
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Reviewed-by: Johannes Kron <kron@webrtc.org>
> Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
> Cr-Commit-Position: refs/heads/main@{#47656}

Bug: webrtc:455813732
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I79249c1d2e5890ff3dd6c396d793f8ddf45d6dde
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470960
Owners-Override: Johannes Kron <kron@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#47661}