Delete UnitBase::Clamp and Clamped in favor of std::clamp

These functions predate c++17 std::clamp and are now obsolete in favor
of a standard way to write clamping.

Bug: None
Change-Id: If7d5eeeabf6bccca38b558051b89be2e44d99451
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/423600
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46152}