main, model: don't 'initialize' system_context

main, model: don't 'initialize' system_context

I was unnecessarily giving main.cc control of the system_context variable,
since I thought I needed to use getcontext() before entering the model checker.
However, the structure of the runtime scheduling is such that this
"initialization" would be obliterated by the first swapcontext() call
(switching form system-context to user-context).

So, the point is that the model-checker can just declare its own
("uninitialized") context that will be initialized as soon as the model-checker
makes a thread swap. Thus, I remove the external interface for initializing the
context (set_system_context()).