all: simplify unsafe.Slice usage in {Byte,UTF16}PtrToString

On windows, use unsafe.Slice instead of unsafeheader as already the case
for unix and plan9.

The pointers are already *byte/*uint16, so the type conversion can be
omitted as well.

Change-Id: Ida7264cc0c1948bf563ed91d51e637edcdafb77a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/430515
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

windows/syscall_windows.go

@@ -138,13 +138,7 @@ func UTF16PtrToString(p *uint16) string {
 		ptr = unsafe.Pointer(uintptr(ptr) + unsafe.Sizeof(*p))
 	}
 
-	var s []uint16
-	h := (*unsafeheader.Slice)(unsafe.Pointer(&s))
-	h.Data = unsafe.Pointer(p)
-	h.Len = n
-	h.Cap = n
-
-	return string(utf16.Decode(s))
+	return string(utf16.Decode(unsafe.Slice(p, n)))
 }
 
 func Getpagesize() int { return 4096 }