4136 by Inlead: Use core function and not variable-contrib function.

[inlead]

Link to issue

https://platform.dandigbib.org/issues/4136

Description

This change-set prevents DBCs environment to fail upon deployment of ddb_cp-active clients.
Apparently variable_delete() is a function from within the variable-contrib modul and not Drupal core. Since the variable module is only used by the ddb_cp module, it was only reproducible on "Webmaster"-libraries.

Checklist

• My complies with our coding guidelines.
• My code passes our static analysis suite. If not then I have added a comment explaining why this change should be exempt from the code standards and process.
• My code passes our continuous integration process. If not then I have added a comment explaining why this change should be exempt from the code standards and process.

Additional comments or questions

This only solves the problem and does not prevent any future errors of similar kind in regards to deployment.