Stop checking EventBase::runInEventBaseThread result
Stop checking EventBase::runInEventBaseThread result

Summary:
[Folly] Stop checking `EventBase::runInEventBaseThread` result, as the function will soon be changed not to return any result.

It returned `false` when failing to enqueue a task. But it cannot really fail anyway besides allocation failure, unless in the `EventBase` destructor and while draining and the `AlwaysEnqueue` variant is called.

Reviewed By: andriigrynenko

Differential Revision: D14254969

fbshipit-source-id: a6a9199cbafa18b61488a240e4318ce946953f51

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter