Remove Initialize method from JsepSessionDescription

This change removes the deprecated `Initialize` method, which allowed
for a problematic two-step initialization. This pattern prevented class
members from being `const` and could lead to objects in an indeterminate
state if called more than once.

By removing this method, initialization is now enforced exclusively
through the constructor. This change improves const-correctness by
making the session description, ID, and version immutable after
construction, leading to more robust and predictable behavior.

Bug: webrtc:442220720
Change-Id: I61b258f8d00491f781284ce4d8c40fc98540810e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/408882
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45617}