Remove pop-up test for unscopable popup attr

There was a previous web compat issue with event handler attributes
calling functions called "popup()". As a result, we made the `popup`
attribute unscopable, so this would not break the web. However,
with the change of name to `popUp` for the IDL attribute, this
compat issue likely doesn't exist, for the same reasons that this
rename fixed the general compat issue [1].

This CL removes the test for unscopable. The [2] CL actually made
the attribute scopable.

[1] openui/open-ui#546
[2] https://chromium-review.googlesource.com/c/chromium/src/+/3954386

Bug: 1307772
Change-Id: I603e75338e1ccb925e6f145870fa6fecc68c61fd