buffer: remove Uint8Array check

This makes write[U]Int* operations on Buffer with `noAssert=false` about 3
times faster.

PR-URL: https://github.com/nodejs/node/pull/11324
Refs: https://github.com/nodejs/node/issues/11245


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>