Use config()->delete() instead of config_clear() in .install file

[alanmels]

Please read backdrop/backdrop-issues#4186 (comment) to see that replacing Drupal 7's variable_del() function in .install files is not sufficient. Either config_clear() should be used in the .module file to declare variables and then the variable_del() function in .install should be dropped, or the following function:

/**
 * Implements hook_uninstall().
 */
function $mymodule_uninstall() {
  config('mymodule.settings')->delete();
}

should be used instead to get the uninstalled module's configuration file actually deleted.

[bugfolder]

The call to config('mymodule.settings')->delete() is superfluous. If a module's config file is properly declared by hook_config_info(), then it will be automatically deleted when the module is uninstalled.

Prior to 2021-03-14, there was a bug in the automatic generation of hook_config_info() that resulted in the config file not being properly declared, and thus, being left as a residue when the module was uninstalled. See #28 and associated PR.

Since this comment was written prior to that, that would explain the behavior reported in the comment above. But it's now been fixed.