unique: fix inconsistent panic prefix in canonmap cleanup path
Other panic messages in the file use the "unique.canonMap"
prefix, but this one used "internal/sync.HashTrieMap",
looks like it was copied from the "internal/sync" package.
Change-Id: Ic3e85154eb5a593d41c19013d5696afed2178b7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/713660
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: David Chase <drchase@google.com>