Don't assume the type of Time::point

But instead use the proper definition. Also
rewrite chrono functions while there.

No functional change.

src/misc.cpp

@@ -17,7 +17,6 @@
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
-#include <chrono>
 #include <fstream>
 #include <iomanip>
 #include <iostream>
@@ -27,7 +26,6 @@
 #include "thread.h"
 
 using namespace std;
-using namespace std::chrono;
 
 namespace {
 
@@ -125,13 +123,6 @@ const string engine_info(bool to_uci) {
 }
 
 
-/// Convert system time to milliseconds. That's all we need.
-
-Time::point Time::now() {
-  return duration_cast<milliseconds>(steady_clock::now().time_since_epoch()).count();
-}
-
-
 /// Debug functions used mainly to collect run-time statistics
 
 void dbg_hit_on(bool b) { ++hits[0]; if (b) ++hits[1]; }