Retire the transition scaffolding (#515)

Two files on this branch describe a repository that no longer exists.

ZENOH_FLAT_TRANSITION.md was the integration branch's own bookkeeping — an
architecture sketch and a table of the constituent PRs. The table stopped at
#484 and its links still pointed at the ZettaScaleLabs forks the work has
since left, so it now misinforms rather than informs; the same content, kept
current, lives in the #482 description. It was always meant to go when the
branch merges.

rust-toolchain.toml pinned 1.97.1 for a repository that builds no Rust. CI
installs its components explicitly (`--toolchain 1.93.0`) and runs every cargo
command inside the sibling zenoh-flat-jni checkout, where that repository's own
toolchain file applies, so this pin selects nothing and can only mislead
whoever reads it next.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: milyin