Leftover HPX_REGISTER_ACTION_DECLARATION_2

[AntonBikineev]

After moving action serialization to new library we don't need to have this macro anymore.
https://github.com/STEllAR-GROUP/hpx/blob/master/hpx/runtime/actions/basic_action.hpp#L504-L505

[hkaiser]

What about HPX_REGISTER_ACTION_DECLARATION_, HPX_REGISTER_ACTION_DECLARATION_1 and possibly others?

[hkaiser]

The actual question is: should we completely remove the two-argument version of HPX_REGISTER_ACTION_DECLARATION? This would be a breaking change and would require modification of a lot of code out there.

[sithhell]

I actually like the binary version. It creates a nice symmetry between the declaration and definition, even though the name is not used when the macro is expanded, it serves as a nice documentation.

[hkaiser]

I'll close this for now as the consensus seems to be to leave the macro in place. Please reopen if there are concerns.