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 <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: tklauser

unix/mksyscall.go

@@ -363,7 +363,6 @@ func main() {
 				text += fmt.Sprintf("func libc_%s_trampoline()\n", libcFn)
 				// Assembly trampoline calls the libc_* function, which this magic
 				// redirects to use the function from libSystem.
-				text += fmt.Sprintf("//go:linkname libc_%s libc_%s\n", libcFn, libcFn)
 				text += fmt.Sprintf("//go:cgo_import_dynamic libc_%s %s \"/usr/lib/libSystem.B.dylib\"\n", libcFn, libcFn)
 				text += "\n"
 			}

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) {