Remove unused SKIP_VARIABLE_CHECKS

Migrated issue

Reported by: jonathan1055

Related to !90 (merged)

I asked on Slack https://app.slack.com/client/T06GX3JTS/CGKLP028K

jonathan: "There is a variable SKIP_VARIABLE_CHECKS in the includes/include.drupalci.variables.yml but this is never referred to anywhere else in the codebase. The description is "Set to 1 to skip the variable checks CI job". Did this get in early, before work was done? or has something been removed but the variable still listed? Or maybe somehow this is used in other codebase but needs to be defined in gitlab_templates?"

moshe: "I think that job got removed, so we should remove the variable"