cmd/compile/internal/types: remove unused Tie method

Once upon a time, this was used by package walk to compute the
"conv[TIE]2[TIE]" function names, etc.; but it seems like those
callers have all changed to directly specializing on IsInterface and
IsEmptyInterface instead.

Change-Id: I629cdf076a09e7255ae293b8f879db0cdcf4de5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/345793


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>