x/tools: remove dead code

These unused functions and methods were discovered by a (work in
progress) dead function analyzer.

Change-Id: Iaa9c5fd4f06bdcd5d7c6e194bcc24d6b37df8f4c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/639675
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/jsonrpc2/jsonrpc2.go

@@ -112,15 +112,6 @@ func (a *async) done() {
 	close(a.ready)
 }
 
-func (a *async) isDone() bool {
-	select {
-	case <-a.ready:
-		return true
-	default:
-		return false
-	}
-}
-
 func (a *async) wait() error {
 	<-a.ready
 	err := <-a.firstErr