[Code Health] Make gpu::SyncToken trivially copyable

SyncToken's copy constructor and copy assignment operator are defaulted
out of line in sync_token.cc. That makes them user-provided, so
SyncToken is not trivially copyable and
base::byte_span_from_ref(base::allow_nonunique_obj, ...) rejects it.

Remove both declarations so the compiler generates trivial copy and move
operations. The chromium-style plugin no longer requires out-of-line
copy operations here: it now counts the CommandBufferId member as
trivial.

With a trivial copy, clang now flags the unused SyncToken copy in
TaskGraph::Sequence::FinishTask() with -Wunused-variable, so remove it.

No behavior change.

TAG=agy

Bug: 490869816
Change-Id: I97f3012cb4b41966657a7f6ac2d499d474f2da5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8598756
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ali Hijazi <ahijazi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1720846}
NOKEYCHECK=True
GitOrigin-RevId: 5263cff9658a9d3c4a8a0890350821a33cc16bd6