unix: remove ineffectual //go:linkname directives

For golang/go#42938

Change-Id: I9d68a2436f3ce7402b3c6517a630d0272cf39071
Reviewed-on: https://go-review.googlesource.com/c/sys/+/274573
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

unix/syscall_darwin.1_13.go

@@ -26,7 +26,6 @@ func fdopendir(fd int) (dir uintptr, err error) {
 
 func libc_fdopendir_trampoline()
 
-//go:linkname libc_fdopendir libc_fdopendir
 //go:cgo_import_dynamic libc_fdopendir fdopendir "/usr/lib/libSystem.B.dylib"
 
 func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {