The Elasticsearch startup scripts contain checks for the presence of
support for environment variables that were removed in the 5.x
series. These checks warn the user and fail the script if any of the
unsupported environment variables are present. This was provided as
migration step from 2.x to 5.x so that we were not just silently
ignoring environment variables that were previously set. This commit
removes these checks, as upgrades from 2.x to 6.x are not supported.
Relates #20404
f58a33eb90