Remove unused variables

Detected thanks to the RSpec/LetSetup rule after updating rubocop-rspec.

spec/shared/models/acts_as_imageable.rb

@@ -1,6 +1,5 @@
 shared_examples "acts as imageable" do |imageable_factory|
-  let!(:image)                  { build(:image, imageable_factory.to_sym) }
-  let!(:imageable)              { image.imageable }
+  let!(:image) { build(:image, imageable_factory.to_sym) }
 
   it "is valid" do
     expect(image).to be_valid

spec/shared/models/document_validations.rb

@@ -2,7 +2,6 @@ shared_examples "document validations" do |documentable_factory|
   include DocumentablesHelper
 
   let!(:document)               { build(:document, documentable_factory.to_sym) }
-  let!(:documentable)           { document.documentable }
   let!(:maxfilesize)            { max_file_size(document.documentable.class) }
   let!(:acceptedcontenttypes)   { accepted_content_types(document.documentable.class) }
 

spec/shared/models/image_validations.rb

@@ -2,7 +2,6 @@ shared_examples "image validations" do |imageable_factory|
   include ImageablesHelper
 
   let!(:image)                  { build(:image, imageable_factory.to_sym) }
-  let!(:imageable)              { image.imageable }
   let!(:acceptedcontenttypes)   { imageable_accepted_content_types }
 
   it "is valid" do