[FIX] web_editor: properly call on_remove method of snippet options

The snippet options can define an on_remove method which was supposed
to be called when the snippet is removed. This was not the case since
the method was written so that the options were removed before the
on_remove method was called... (Un)fortunately, the on_remove callbacks
were not doing any critical job in current implementation. In saas-13
however, some elements will rely on this.