Remove the hook_update_7xyz() implementations

[avpaderno]

Since the modules that are part of this project are not supposed to be installed and enabled in a site when it is migrated from Drupal 7 to Backdrop, the hook_update_N() implementations for Drupal 7 should be removed.
If a development site is migrated from Drupal 7 to Backdrop, the example modules should first be uninstalled, which is also necessary because the example modules do not migrate the values set with variable_set(). (None of the example modules calls update_variable_get() in an update hook.)