go/types, types2: remove internal constant enableInterfaceInference

go/types, types2: remove internal constant enableInterfaceInference

We're not going to disable this functionality as it would
invalidate code that now type-checks with type inference
based on this mechanism.

Change-Id: Ib11059e2f70a5a209693b06f754cba862e3650ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/514715
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>