[cssom] Please remove _camel_cased_attribute and friends from the IDL index

[Ms2ger]

https://drafts.csswg.org/cssom/#idl-index

Having those listed makes it more annoying to update the idlharness test.

CC @zcorpan

[zcorpan]

@tabatkins how do I do this?

[csnardi]

These no longer appear to show up in the IDL index, so I think this can be closed. (also related, #2487)