go/types: remove tparamIsIface flag and corresponding dead code

go/types: remove tparamIsIface flag and corresponding dead code

This is a port of CL 363654 from types2 to go/types.

Change-Id: I64041615ccc7f11f2e4ae395b063ec5141ccf2cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/364896
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>