Update for new warn-by-default clippy lints

Remove an import that is actually unused, and silence the approximate
constant lint because it isn't always correct here.

Author: tgross35

libm/src/math/mod.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::approx_constant)] // many false positives
+
 macro_rules! force_eval {
     ($e:expr) => {
         unsafe { ::core::ptr::read_volatile(&$e) }