Remove stuff that has not been needed for a while

During the migration from Ruby 2.6 to Ruby 2.7 (first Ruby which
includes Bundler 2.x by default), this spec caused some issues because
dependabot would issue a call to the RubyGems dependency API to figure
out information about Bundler. But this is not needed these days and it
probably wasn't since the beginning, because dependabot should always
use the running Bundler version information in this case, no need to
query the API.

bundler/spec/dependabot/bundler/update_checker/version_resolver_spec.rb

@@ -47,7 +47,6 @@
   end
   let(:source) { nil }
   let(:requirement_string) { ">= 0" }
-  let(:rubygems_url) { "https://index.rubygems.org/api/v1/" }
 
   describe "#latest_resolvable_version_details" do
     subject { resolver.latest_resolvable_version_details }
@@ -142,11 +141,6 @@
 
           let(:dependency_files) { bundler_project_dependency_files("bundler_specified") }
 
-          before do
-            stub_request(:get, rubygems_url + "versions/bundler.json").
-              to_return(status: 200, body: fixture("rubygems_responses", "versions-bundler.json"))
-          end
-
           it "returns nil as resolution returns the bundler version installed by core" do
             expect(subject).to be_nil
           end