Remove the quad_precision_float feature gate

The f128 type has very little support in the compiler and the feature is
basically unusable today. Supporting half-baked features in the compiler can be
detrimental to the long-term development of the compiler, and hence this feature
is being removed.

src/libsyntax/ast_util.rs

@@ -179,7 +179,6 @@ pub fn float_ty_to_str(t: FloatTy) -> String {
     match t {
         TyF32 => "f32".to_string(),
         TyF64 => "f64".to_string(),
-        TyF128 => "f128".to_string(),
     }
 }