refactor: move Boost datetime usage to wallet

This means we don't need datetime in a --disable-wallet build, and it
isn't included in the kernel.

src/util/time.h

@@ -109,7 +109,6 @@ T GetTime()
  */
 std::string FormatISO8601DateTime(int64_t nTime);
 std::string FormatISO8601Date(int64_t nTime);
-int64_t ParseISO8601DateTime(const std::string& str);
 
 /**
  * Convert milliseconds to a struct timeval for e.g. select.