tracing: fix -Wunused-private-field warning

Remove `external_buffer_` from `InternalTraceBuffer` as it seems not to
be used anywhere.

PR-URL: https://github.com/nodejs/node/pull/10416


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