net: use strict comparisons for fd

This commit removes an eslint-disable comment, and moves file
descriptor checks to use strict equality. These checks were
using loose equality to support numeric and string file
descriptors. However, d9e95d89
introduced strict validation, making the loose equality checks
redundant.

PR-URL: https://github.com/nodejs/node/pull/25014


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>