Remove SimpleStringBuilder and move StringBuilder impl to cc

- Removed deprecated SimpleStringBuilder class.
- Moved StringBuilder implementation to CC file.
- Updated documentation to recommend StringBuilder.

Some benefits of moving parts of the implementation to the cc file:

- Help with enforcing correct build dependencies
  (including in downstream code)
- Moves into the direction of making absl::StrCat an implementation
  detail of StringBuilder (helps with iwyu). This CL doesn't remove
  the include though for other reasons, just a step.
- Small binary size benefits.

Bug: none
Change-Id: I4b9efdba45c3c4923233188ff38476e0ca2257c1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/466380
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47544}