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

spec/models/abilities/common_spec.rb

@@ -14,6 +14,7 @@ describe Abilities::Common do
   let(:own_debate)   { create(:debate,   author: user) }
   let(:own_comment)  { create(:comment,  author: user) }
   let(:own_proposal) { create(:proposal, author: user) }
+  let(:own_legislation_proposal) { create(:legislation_proposal, author: user) }
 
   let(:accepting_budget) { create(:budget, :accepting) }
   let(:reviewing_budget) { create(:budget, :reviewing) }
@@ -167,6 +168,9 @@ describe Abilities::Common do
     it { should_not be_able_to(:destroy, proposal_document)      }
   end
 
+  it { should_not be_able_to(:edit, own_legislation_proposal) }
+  it { should_not be_able_to(:update, own_legislation_proposal) }
+
   describe "proposals dashboard" do
     it { should be_able_to(:dashboard, own_proposal) }
     it { should_not be_able_to(:dashboard, proposal) }