SI-8132 Fix false "overrides nothing" for case class protected param

Case class parameters that are less-than-public have an accessor
method created. In the enclosed test, we saw:

    case class G extends AnyRef with T with Product with Serializable {
      override <synthetic> <stable> <caseaccessor> def s$1: String = G.this.s;
      <caseaccessor> <paramaccessor> private[this] val s: String = _;
      override <stable> <accessor> <paramaccessor> protected def s: String = G.this.s;
      ...
    }

This commit removes the OVERRIDE flag from the accessor method,
which avoids the spurious "overrides nothing" error.
Merge pull request #3369 from retronym/ticket/8132

SI-8132 Fix false "overrides nothing" for case class protected param