Remove obsolete SNOWPLOW_MICRO env variables

What does this Merge Request do and why?

Issue gitlab#358623 (closed) has introduced a new way of setting up snowplow micro - the ENV values are no longer needed.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Related to gitlab#365168 (closed)