cmd/compile/internal/types2: remove structuralString in favor of inlined code

structuralString was used only in one place (for built-in copy).
Remove it in favor of custom and more efficient inlined code.

Follow-up on feedback received for CL 363075.

Change-Id: Ic5857c47255c5c712be7971aae4542fef9960fe6
Reviewed-on: https://go-review.googlesource.com/c/go/+/363154


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