target/arc: simplify arc_config_step()

[artemiy-volkov]

In target/arc.c, the function arc_config_step() looks as if it's trying to toggle between single-step and normal operation mode. This is misleading; writing to DEBUG.IS will always result in executing one instruction and there is no concept of enabling/disabling the single-step mode. It appears that a parameter and ~80% of the function body can be removed.