remove const_fn_floating_point_arithmetic feature

This can be removed now that the feature is stabilized.

tests/mod.rs

@@ -1,6 +1,5 @@
 #![allow(incomplete_features)]
 #![feature(generic_const_exprs, adt_const_params)]
-#![feature(const_fn_floating_point_arithmetic)]
 
 pub mod example_system;
 pub mod utils;