Remove unstable dependency on Drupal's system module

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Problem/Motivation

Crouton is using an internal API of Drupal's "system" module to create its configuration menu structure.

Proposed resolution

Completely remove any configuration menu links provided by Crouton and all supporting infrastructure to eliminate this dependency on Drupal's internal API.

User interface changes

Crouton will no longer create a menu item in the administration's configuration menu. Instead, you can find the module's settings by navigating to Drupal's module installation interface and clicking "Configure" after locating Crouton in the list.