net: simplify net.Socket#end()

`writable` is already set by the streams side, and
there is a handler waiting for the writable side to finish
which already takes care of the other cleanup code that
was previously there; both of these things can therefore be removed.

PR-URL: https://github.com/nodejs/node/pull/18708


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>