test: remove superfluous checks in test-net-reconnect-error

The process.on('exit', ...) checks duplicate the work of
`common.mustCall()` and are superfluous. Remove them.

Signed-off-by: Rich Trott <rtrott@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/32120


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>