Uninstalling the module leaves variables in the database

Problem/Motivation

Uninstalling the module leaves the datalayer__ variables in the database, despite its hook_uninstall() trying to delete them.
The code in the hook is wrong and in fact raises the following errors:

Illegal offset type in unset bootstrap.inc:1273
Illegal offset type in unset bootstrap.inc:1273
Illegal offset type in unset bootstrap.inc:1273

The implementation is quite dangerous as the hook tries to delete variables named with the content of a datalayer variable. For example if the variable schema contains a datalayer_ prefixed variable with content "site_frontpage", the site frontpage is lost.

Proposed resolution

Fix hook_uninstall().