repl: remove obsolete buffer clearing

It is checked if an command buffer exists or not. This code branch
can only be reached if none exist, so there's no need to clear that
buffer again.

PR-URL: https://github.com/nodejs/node/pull/25731


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>