test: remove common.localhostIPv6

common.localhostIPv6 is almost entirely unused and is unnecessary.
Remove it.

PR-URL: https://github.com/nodejs/node/pull/34373


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>