Remove metrics instrumentation for _get_next_execution method.
Remove metrics instrumentation for _get_next_execution method.

This method is called every 1ms by default so instrumentation causes a
lot of overhead and too much noise if log level is set to DEBUG.

nzlosh/st2,Plexxi/st2,StackStorm/st2,StackStorm/st2,StackStorm/st2,Plexxi/st2,nzlosh/st2,Plexxi/st2,Plexxi/st2,nzlosh/st2,nzlosh/st2,StackStorm/st2