remove some no-longer-needed allow(const_err)

Author: RalfJung

tests/run-pass/coerce_non_capture_closure_to_fn_ptr.rs

@@ -1,7 +1,4 @@
-// allow(const_err) to work around a bug in warnings
-#[allow(const_err)]
 static FOO: fn() = || { assert_ne!(42, 43) };
-#[allow(const_err)]
 static BAR: fn(i32, i32) = |a, b| { assert_ne!(a, b) };
 
 // use to first make the closure FnOnce() before making it fn()