go/types, types2: no need to revert tparam renaming in inference results

This is a follow up to CL 385494. In early patch sets of that CL,
renamed type parameters were substituted in arguments, which meant that
they could leak into the inference results. However, we subsequently
realized that we could instead substitute in the signature parameters.
In this case it is not possible for the substituted type parameters to
appear in the resulting type arguments, so there is no need to
un-substitute.

Change-Id: I4da45b0b8d7ad809d0ddfa7061ae5f6f07895540
Reviewed-on: https://go-review.googlesource.com/c/go/+/385574


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>