Remove support for IE conditional scripts and styles

[joedolson]

Returns false if conditional scripts or styles are identified; updates comments; removes files that were only loaded conditionally.

Trac ticket: https://core.trac.wordpress.org/ticket/63821

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[sirreal]

I scanned this and didn't t see any _doing_it_wrong_ or similar notices in this change. Did you add any? Should some notice be included?

What if each class handed some notice and ignored conditionals in ::add_data()? They could avoiding adding conditional scripts/styles at all. It would save handling conditional scripts and styles when printing.

We could consider doing that in WP_Dependencies::add_data() as well, although there's a small risk of affecting other subclasses. I scanned a quick wpdirectory search and it didn't seem likely to be a problem, but it would be safer to do it in the script and style classes.

[joedolson]

I was thinking about handling this in ::add_data(), but if we exit before adding the data, then the related script will still be handled, and will be printed to the page, just without conditionals. Handling it in do_item() allows us to also ignore the script itself and its dependencies.

I guess we could de-register the handle in add_data(), but that feels dirty, somehow.

Can still handle a deprecation notice in add_data(), though.