Replace GetSeconds() implementation by utility function of OSC API

In ConvertEnvironment.cpp we had to implement the GetSeconds function ourselves because the utility function "static long long NET_ASAM_OPENSCENARIO::DateTime::GetSeconds(const NET_ASAM_OPENSCENARIO::DateTime::DateTime& dateTime)" was not exported from the OSC API.
With the OSC API v1.4.0-b2 the function is now exported and can be used, so we don't need our reimplementation anymore.