[dev.link] cmd/link: remove safe mode

[dev.link] cmd/link: remove safe mode

Safe mode in the compiler is removed in CL 142717 in Go 1.12. I
think we can delete safe mode from the linker as well.

Change-Id: I201e84fca3a566a1bb84434ab4d504516160ac4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/236117
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>