gs-app-row: Remove ‘show-folders’ functionality

It was disabled by default, and the only call to gs_app_row_set_show_folders() in the codebase also disabled it.

The show-folder-management GSetting has previously been removed in 67a5bff9, so that was also causing this functionality to be disabled.

Signed-off-by: Philip Withnall withnall@endlessm.com