cmd/compile/internal/types2: partial revert of incorrect unification "fix"

The "fix" (CL 352832) for #48619 was incorrect and broke
the unification algorithm in some cases (e.g., #48695).

This CL reverts the changes made by CL 352832 to unify.go,
and comments out code in corresponding tests.

As a result, #48695 will be fixed, and we will re-open #48619.

Fixes #48695.
For #48619.
For #48656.

Change-Id: I91bc492062dbcc8dae7626f6b33f6dfabf48bcb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354690


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