[rubygems/rubygems] Remove unnecessary code from remote fetcher tests

This code is not necessary and has the bad side effect of causing
test failures in cascade once a single test inside the file fails.

See https://github.com/rubygems/rubygems/runs/538978838 for an example.

https://github.com/rubygems/rubygems/commit/2291d32915

test/rubygems/test_gem_remote_fetcher.rb

@@ -108,10 +108,7 @@ PeIQQkFng2VVot/WAQbv3ePqWq07g1BBcwIBAg==
     @server_uri = base_server_uri + "/yaml"
     @server_z_uri = base_server_uri + "/yaml.Z"
 
-    # REFACTOR: copied from test_gem_dependency_installer.rb
-    @gems_dir = File.join @tempdir, 'gems'
     @cache_dir = File.join @gemhome, "cache"
-    FileUtils.mkdir @gems_dir
 
     # TODO: why does the remote fetcher need it written to disk?
     @a1, @a1_gem = util_gem 'a', '1' do |s|