[css3-values] Remove issue about attr() auto-inferring type

The attr() section of CSS3 Values
<http://dev.w3.org/csswg/css3-values/#attr> currently has an issue
asking if we can just infer the type of the value if attr() is used on
a property that only accepts a single type.

This would be a bad idea.  It means that if a property is ever defined
to take a single type of value, we are then unable to change it in the
future to take two or more types of values without possible breaking
attr() use.

I think the note should just be removed.

~TJ