buffer: remove obsolete and confusing comment
Checklist
Affected core subsystem(s)
buffer
Description of change
This comment applied to a line that was removed in https://github.com/nodejs/node/commit/dd67608bfdb3bef6e06a1965ecbfa51658c61b1b and is no longer relevant.
...Or so it seems. The reason for keeping it was possibly discussed in https://github.com/nodejs/node-private/pull/30, but I don't have access to it.