Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

internal/localrpc/localrpc.go

@@ -128,11 +128,8 @@ func readRequest(r *textproto.Reader) (string, string, error) {
 		return "", "", err
 	}
 
-	sp := strings.SplitN(line, " ", 2)
-	if len(sp) == 1 {
-		return sp[0], "", nil
-	}
-	return sp[0], sp[1], nil
+	name, params, _ := strings.Cut(line, " ")
+	return name, params, nil
 }
 
 func writeError(tr *trace.Trace, tconn *textproto.Conn, err error) {