buffer: remove noAssert argument

noAssert=true is intrinsically unsafe. Always-on checking does cause a performance regression but some of that can be regained by open-coding the checks (which this pull request doesn't do but can be done at a later time if so desired.)

What's more, I don't think performance is ultra-critical for these methods. If you want to squeeze out every last ounce of oomph, you don't call out to generic kitchen-sink methods, you open-code loads and stores directly in your parser or generator. I can see no good reason for leaving it in and that is why this commit takes it out.

Refs: #8724 (closed) (https://github.com/nodejs/node/issues/8724#issuecomment-289573174 is relevant vis-a-vis performance.)