assert: remove internal errorCache property

The internal assert module exposed an errorCache property solely for
testing. It is no longer necessary. Remove it.

PR-URL: https://github.com/nodejs/node/pull/23304


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>