src: remove usage of AllocatedBuffer from src/node_buffer.cc

Since AllocatedBuffer is just a thin wrapper around v8::BackingStore, we
should prefer using v8::BackingStore directly.

Refs: https://github.com/nodejs/node/blob/52abf271c563ddffdc93b444ea05e5347a7f2784/src/allocated_buffer.h#L30-L31



Signed-off-by: Darshan Sen <darshan.sen@postman.com>

PR-URL: https://github.com/nodejs/node/pull/39941


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>