Don't warn unused param of implicit marker

[som-snytt]

Reproduction steps

Scala version: 2.13.12

//> using options -Wunused:implicits

trait CanEqual[A, B]

class EqTest {
  implicit class ArrowAssert[A](a: A) {
    def ==>[B](b: B)(implicit ev: CanEqual[A, B]): Boolean = a == b // no warn, no declared members
  }
}

Problem

Warning is omitted for certain well-known type classes (=:=, etc). It would be nice to not warn if the typeclass declares no members, as no usage is meaningful except passing it down the stack.

[som-snytt]

Thanks, I needed something to do after today's wordle in 3. If only I'd done it in time for 2.13.13.

[som-snytt]

I was nerd-sniped into fixing this twice.