Remove constant unix cgo (#125)

* Revert "Add ExtrinsicSignatureV5 (#68)"

This reverts commit 461cf42.

* rm const unix cgo

Author: mikiquantum

gethrpc/constants_unix.go

@@ -14,21 +14,8 @@
 // You should have received a copy of the GNU Lesser General Public License
 // along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
 
-// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris
-
 package rpc
 
-/*
-#include <sys/un.h>
-
-int max_socket_path_size() {
-struct sockaddr_un s;
-return sizeof(s.sun_path);
-}
-*/
-// import "C"
-
 var (
-	// max_path_size = C.max_socket_path_size()
 	max_path_size = 108
 )