src: remove outdated todo from node_crypto.cc

TODO comment from node_crypto is no longer relevant. Unification of
commented code and string_bytes code would bloat the latter. Methods
for hex encoding produce different output in both files (crypto adds
colon and uses uppercase letters.) Common path between those two is
very limited now.

PR-URL: https://github.com/nodejs/node/pull/15104


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>