_Z_OWNED_FUNCTIONS_SYSTEM_IMPL macro is dummy

[milyin]

Describe the release item

This macro implements common functions like z_check, z_null etc. But as this macro is used on structure with _val field of unknown (platform-dependent) type, like _z_task_t which is defined differently for each platform, this macro just provides empy implementation for these functions.
This makes our API incorrect. We guarantee that z_drop after z_null is safe, as well as double z_drop. But this is not the case for this implementation.

See also discussion in eclipse-zenoh/zenoh-c#492

[sashacmc]

@milyin please assign it to me

[milyin]

The proposed solution is just don't support double drop for these types. This is necessary for C++ wrapper only, but these types are not supposed to be used in Zenoh C++ - the C++ have it's own support for these primitives (thread, mutex, etc).

The only thing to do then is just to remove the dummy functions z_null / z_check for these types. This will be the signal that these types are specific and doesn't have "gravestone" state