Remove duplicate declaration of test mail delivery

Our test environment sets the mail delivery method already. We don't
have to do it in individual spec files.

spec/lib/spree/core/mail_interceptor_spec.rb

@@ -15,10 +15,6 @@ describe Spree::OrderMailer do
   end
 
   context "#deliver" do
-    before do
-      ActionMailer::Base.delivery_method = :test
-    end
-
     after { ActionMailer::Base.deliveries.clear }
 
     it "should use the from address specified in the preference" do

spec/mailers/order_mailer_spec.rb

@@ -158,7 +158,6 @@ describe Spree::OrderMailer do
     end
 
     before do
-      ActionMailer::Base.delivery_method = :test
       ActionMailer::Base.deliveries = []
     end
 

spec/mailers/user_mailer_spec.rb

@@ -10,7 +10,6 @@ describe Spree::UserMailer do
   end
 
   before do
-    ActionMailer::Base.delivery_method = :test
     ActionMailer::Base.deliveries = []
   end