Remove unused mock from test (#3793)

Just a small cleanup: the `render_document_sidebar_partial` helper
was removed in #3334, so there is no need to mock it in our test anymore.

Author: sandbergja

spec/views/catalog/show.html.erb_spec.rb

@@ -9,7 +9,6 @@
     allow(presenter).to receive(:html_title).and_return('Heading')
     allow(document).to receive(:more_like_this).and_return([])
     allow(view).to receive_messages(has_user_authentication_provider?: false)
-    allow(view).to receive_messages(render_document_sidebar_partial: "Sidebar")
     allow(view).to receive_messages(current_search_session: nil, search_session: {})
     assign :document, document
     allow(view).to receive_messages(document_presenter: presenter, action_name: 'show', blacklight_config: blacklight_config)