[rubygems/rubygems] Suppress warning during test runner.

ruby core repository explicitly removed this test file at
c945a849cb
for avoid to show warning.

https://github.com/rubygems/rubygems/commit/ee78e64b50

test/rubygems/test_gem_package_task.rb

@@ -9,10 +9,6 @@ rescue LoadError => e
   raise unless e.path == "rake/packagetask"
 end
 
-unless defined?(Rake::PackageTask)
-  warn "Skipping Gem::PackageTask tests.  rake not found."
-end
-
 class TestGemPackageTask < Gem::TestCase
   def test_gem_package
     original_rake_fileutils_verbosity = RakeFileUtils.verbose_flag