go.tools/go/types: fix incorrect receiver in (*Selection).Type().

Method expressions T.f are reported as having type (T)func(T),
i.e. T appears twice, as a receiver and a regular parameter.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/96780044

go/types/selection.go

@@ -84,6 +84,7 @@ func (s *Selection) Type() Type {
 		// TODO(gri) Similar code is already in call.go - factor!
 		sig := *s.obj.(*Func).typ.(*Signature)
 		arg0 := *sig.recv
+		sig.recv = nil
 		arg0.typ = s.recv
 		var params []*Var
 		if sig.params != nil {