Remove unnecessary runtime check
Remove unnecessary runtime check

Summary:
It's nice to be on a safe side, but this is completely redundant. `static_assert`s above are enough.
We'll do up to 9 iterations of the while loop, thus shift number by 63 bits, so we'll have only one bit remaining that will pass the check.

Reviewed By: jmswen

Differential Revision: D5947313

fbshipit-source-id: d1ab2bc591aaa8ddf9bc9db324187d8cc522b595

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