src: remove unused env strings

user_string:
Usage removed in 4e05952a
onstop_string:
Usage removed in 7b46e177
max_old_space_size_string:
Added in 0df031ac, never used
max_semi_space_size_string:
Added in 0df031ac, never used
enter_string:
Usage removed in eeede3b1

PR-URL: https://github.com/nodejs/node/pull/22137


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>