Drop "using buildpack" build output assertion

The build system now prints a warning after this link as the buildpack URL includes the branch name.

Author: runesoerensen

spec/hatchet/buildpack_spec.rb

@@ -9,7 +9,6 @@
     it 'successfully compiles' do
       app.deploy do |app|
         expect(clean_output(app.output)).to match(Regexp.new(<<~REGEX))
-          remote: -----> Using buildpack: #{DEFAULT_BUILDPACK_URL}
           remote: -----> .NET app detected
           remote: -----> SDK version detection
           remote:        Detected .NET project: `/tmp/build_.*/foo.csproj`