buffer: remove checkNumberType()

checkNumberType() was a very thin wrapper around validateNumber().
This commit removes checkNumberType() and used validateNumber()
directly instead.

PR-URL: https://github.com/nodejs/node/pull/24815


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>