go/types, types2: remove Config.EnableReverseTypeInference flag

go/types, types2: remove Config.EnableReverseTypeInference flag

Proposal #59338 has been accepted and we expect this feature to
be available starting with Go 1.21. Remove the flag to explicitly
enable it through the API and enable by default.

For now keep an internal constant enableReverseTypeInference to
guard and mark the respective code, so we can disable it for
debugging purposes.

For #59338.

Change-Id: Ia1bf3032483ae603017a0f459417ec73837e2891
Reviewed-on: https://go-review.googlesource.com/c/go/+/491798
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>