syscall: remove unused {dragonfly,illumos,solaris}64Bit constants

These are unused since CL 153837. illumos64Bit was added by CL 174457
but was never used.

Change-Id: I34a1bd41cf70f8a07e57f93a71de3c6034fcaf7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/175358
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_unix.go

@@ -21,11 +21,8 @@ var (
 )
 
 const (
-	darwin64Bit    = runtime.GOOS == "darwin" && sizeofPtr == 8
-	dragonfly64Bit = runtime.GOOS == "dragonfly" && sizeofPtr == 8
-	netbsd32Bit    = runtime.GOOS == "netbsd" && sizeofPtr == 4
-	solaris64Bit   = runtime.GOOS == "solaris" && sizeofPtr == 8
-	illumos64Bit   = runtime.GOOS == "illumos" && sizeofPtr == 8
+	darwin64Bit = runtime.GOOS == "darwin" && sizeofPtr == 8
+	netbsd32Bit = runtime.GOOS == "netbsd" && sizeofPtr == 4
 )
 
 func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)