Remove the unneeded isqrt feature gate

[1] has been stabilized so we no longer need to enable it.

[1]: https://github.com/rust-lang/rust/issues/116226

library/compiler-builtins/testcrate/src/lib.rs

@@ -15,7 +15,6 @@
 #![no_std]
 #![cfg_attr(f128_enabled, feature(f128))]
 #![cfg_attr(f16_enabled, feature(f16))]
-#![feature(isqrt)]
 
 pub mod bench;
 extern crate alloc;