compiler: remove references to slicecopy and slicestringcopy

The only calls to the runtime functions were removed in CL 170005.
The slicestringcopy function will be removed in the Go 1.16beta1 release.

Change-Id: Ife8de7067d1094351c53b2f33fafbf37ebdd45c8
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279932
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
2 files changed