src: make EnvDelete behave like the delete operator

According to TC39 specification, the delete
operator returns false or throws
in strict mode, if the property is
non-configurable. It returns true in all other cases.

Process.env can never have non-configurable
properties, thus EnvDelete must always return true. This
is independent of strict mode.

Fixes: https://github.com/nodejs/node/issues/7960
PR-URL: https://github.com/nodejs/node/pull/7975


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>