util: Remove unused MilliSleep

src/util/time.h

@@ -38,8 +38,6 @@ void SetMockTime(int64_t nMockTimeIn);
 /** For testing */
 int64_t GetMockTime();
 
-void MilliSleep(int64_t n);
-
 /** Return system time (or mocked time, if set) */
 template <typename T>
 T GetTime();