Remove unneeded lines restricting permissions

In the past, users had permission to edit their own legislation
proposals. However, that changed in commit ebfa3fb01, where we replaced
the `can` method with `cannot`.

An easier way to remove this permission is to simply remove the whole
statement, since by default users don't have permissions to do anything.

We're also adding a test checking users can't edit their own legislation
proposals, since it was missing.

app/models/abilities/common.rb

@@ -39,9 +39,6 @@ module Abilities
       can [:retire_form, :retire], Proposal, author_id: user.id
 
       can :read, Legislation::Proposal
-      cannot [:edit, :update], Legislation::Proposal do |proposal|
-        proposal.editable_by?(user)
-      end
       can [:retire_form, :retire], Legislation::Proposal, author_id: user.id
 
       can :create, Comment