src: remove unneeded Environment error methods

They seem to have been introduced as "convenience methods" in commit
75adde07 ("src: remove `node_isolate` from source") for reasons I can
only guess at but they can be removed without much hassle.

PR-URL: https://github.com/nodejs/node/pull/8427


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>