Remove text-center class from content-item-button, as it's not longer necessary

Author: toastercup

app/cells/plugins/core/content_item/popup.haml

@@ -3,7 +3,7 @@
 
 = render_label
 %br
-%button.content-item-button.text-center.popup--open
+%button.content-item-button.popup--open
   .content-item-button__select.content-item-button__select--selected
     %i{ class: "material-icons icon" }
       cloud_upload