drop "update" call when a data attribute changes

[kapouer]

This is completely useless: if there is an update fonction on the element, it's a custom element, and custom elements watch their attributes themselves.

[kapouer]

That's done, but there still is a data-* binding, which is somewhat odd:
when a root node view data-xxx attribute changes, and if block.data.xxx exists, the attribute value replaces the block.data.xxx value.
It's cool but feels weird.
See

pagecut/src/specs.js

Line 603 in 284b84f

} else if (record.target == this.dom && record.type == "attributes" && record.attributeName && record.attributeName.startsWith('data-')) {