cmd/link: C is gone, remove check for Go calling C

cmd/link: C is gone, remove check for Go calling C

It looks like the compiler still uses the Cfunc flag for functions
marked as //go:systemstack, but if I'm reading this right, that
doesn't apply here and the linker no longer needs Cfunc.

Change-Id: I63b9192c2f52f41401263c29dc8dfd8be8a901a1
Reviewed-on: https://go-review.googlesource.com/20105
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>