Compiler Attributes: remove comment about sparse not supporting __has_attribute

Sparse supports __has_attribute() since 2018-08-31, so the comment
is not true anymore but more importantly is rather confusing.

So remove it.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>