cmd/link: remove importcycles function
The importcycles method has not been useful since April 2016 when a large code deletion was performed.
The compiler itself provides some protection against import cycles, and the linker does import cycle detection in linksetup -> postorder.
For #57400
Change-Id: I3095bdb3f16a82ba25681bf4a20ceaa3c9613921
GitHub-Last-Rev:
87a46153b136db67675874ffe46e5881b9c756ce
GitHub-Pull-Request: golang/go#57462
Reviewed-on: https://go-review.googlesource.com/c/go/+/459475
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>