cmd/compile: remove unneeded early transforms, with dictionary change

Now that we are computing the dictionary format on the instantiated
functions, we can remove the early transformation code that was needed
to create the implicit CONVIFACE nodes in the generic function.

Change-Id: I1695484e7d59bccbfb757994f3e40e84288759a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/349614


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>