unix: remove darwin direct syscall special case from mksyscall.go

Since CL 250437 this package no longer supports non-libc, direct
syscalls. Thus the corresponding special cases can be removed from
the generating mksyscall.go program.

Change-Id: I54be144de8ffd483f10c7b3a69b0761b83542c55
Reviewed-on: https://go-review.googlesource.com/c/sys/+/251318
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/mksyscall.go

@@ -121,7 +121,7 @@ func main() {
 	}
 
 	libc := false
-	if goos == "darwin" && (strings.Contains(buildTags(), ",go1.12") || strings.Contains(buildTags(), ",go1.13")) {
+	if goos == "darwin" {
 		libc = true
 	}
 	trampolines := map[string]bool{}
@@ -153,11 +153,6 @@ func main() {
 			}
 			funct, inps, outps, sysname := f[2], f[3], f[4], f[5]
 
-			// ClockGettime doesn't have a syscall number on Darwin, only generate libc wrappers.
-			if goos == "darwin" && !libc && funct == "ClockGettime" {
-				continue
-			}
-
 			// Split argument lists on comma.
 			in := parseParamList(inps)
 			out := parseParamList(outps)