SystemTimeStamp used to be an opaque class, as it was provided by
`system_{linux/other/windows}.h`. Because of this, code dealt in
SystemTimeStamp pointers, and getting the current timestamp required a
memory allocation. Now SystemTimeStamp is just a
`std::chrono::time_point`, we can make the code cleaner and faster by
just directly keeping SystemTimeStamp instead of pointers around.
This commit actually fixes two problems causing the leak:
* DestroyTimeStamp not being called in CProfiler (my stupid mistake in 5fea22ff03)
* DestroyTimeStamp leaving null pointers in CSystemUtils::m_timeStamps (this was introduced by @piotrdz long ago when introducing smart pointers)
MrSimbax