remove the const_fn feature gate

it doesn't seem to be needed anymore

Author: japaric

src/generate/device.rs

@@ -45,7 +45,6 @@ pub fn render(d: &Device, target: &Target, nightly: bool, device_x: &mut String)
 
     if *target != Target::CortexM {
         out.push(quote! {
-            #![feature(const_fn)]
             #![feature(try_from)]
         });
     }