Remove literal array mutation that adds an element

It turns out this mutation is not "universal". Meaning not in all
scenarios this extra element triggers an edge case the specs have to
cover.

config/flay.yml

@@ -1,3 +1,3 @@
 ---
 threshold: 16
-total_score: 684
+total_score: 680

lib/mutant/mutator/node/literal/array.rb

@@ -34,7 +34,6 @@ module Mutant
               emit_self(*dup_children)
               mutate_child(index)
             end
-            emit_self(s(:nil), *children)
           end
 
         end # Array

spec/unit/mutant/mutator/node/literal/array_spec.rb

@@ -21,9 +21,6 @@ describe Mutant::Mutator::Node::Literal, 'array' do
 
     # Empty array
     mutations << '[]'
-
-    # Extra element
-    mutations << '[nil, true, false]'
   end
 
   it_should_behave_like 'a mutator'

spec/unit/mutant/mutator/node/literal/empty_array_spec.rb

@@ -1,17 +0,0 @@
-require 'spec_helper'
-
-describe Mutant::Mutator::Node::Literal, 'empty array' do
-  let(:source) { '[]' }
-
-  let(:mutations) do
-    mutations = []
-
-    # Literal replaced with nil
-    mutations << 'nil'
-
-    # Extra element
-    mutations << '[nil]'
-  end
-
-  it_should_behave_like 'a mutator'
-end