Remove unused custom web step

It looks like this web step is no longer used anywhere. It was implemented for the "Feature: inifinite scroll" test, but that no longer exists.

features/step_definitions/custom_web_steps.rb

@@ -167,10 +167,6 @@ Then /^I should see (\d+) contacts$/ do |n_posts|
   has_css?("#people-stream .stream-element", count: n_posts.to_i).should be true
 end
 
-And /^I scroll down$/ do
-  page.execute_script("window.scrollBy(0,3000000)")
-end
-
 Then /^I should have scrolled down$/ do
   expect(page.evaluate_script("window.pageYOffset")).to be > 0
 end