spec_helper: don't try to remove /tmp

The FakeFS spec helper already cleans up the filesystem after each test.
Also, that after helper is executed after FakeFS already deactivated
itself, so in some systems, that can cause the real /tmp to be removed
if the user running the tests has permissions to do so.

spec/spec_helper.rb

@@ -171,7 +171,4 @@ def capture_stdout
   config.before(:each) do
     FileUtils.mkdir_p('/tmp')
   end
-  config.after(:each) do
-    FileUtils.rm_rf('/tmp')
-  end
 end