go/types, types2: remove (internal) exactUnification flag

go/types, types2: remove (internal) exactUnification flag

Neither infer nor infer2 will correctly work if we require
exact unification. Remove the flag and simplify the respective
code.

Change-Id: I329f207f72b6d97fa076f27275481b754e55fecf
Reviewed-on: https://go-review.googlesource.com/c/go/+/464346
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>