[dev.typeparams] cmd/compile/internal/types2: move instance.go contents into named.go (cleanup)

This just moves the code around the instance type into named.go
where it belongs. While at it, also removed some left-over references
to instance types (which are gone). Removed instance.go.

Change-Id: I302a86ca50675b0be54f6138fa47f48f00f9c98f
Reviewed-on: https://go-review.googlesource.com/c/go/+/338469


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>