net: remove redundant code from _writeGeneric()

The encoding is already handled by `Writable.prototype.write()`.

PR-URL: https://github.com/nodejs/node/pull/18429


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>