zkvm: remove cmath

- Remove cmath from zkvm module since cmath was moved to sys and is
shared by all platforms (see #120109)

Author: marijanp

library/std/src/sys/pal/zkvm/mod.rs

@@ -12,8 +12,6 @@ const WORD_SIZE: usize = core::mem::size_of::<u32>();
 pub mod alloc;
 #[path = "../zkvm/args.rs"]
 pub mod args;
-#[path = "../unix/cmath.rs"]
-pub mod cmath;
 pub mod env;
 #[path = "../unsupported/fs.rs"]
 pub mod fs;