Rollup merge of #126552 - fee1-dead-contrib:rmfx, r=compiler-errors

Remove use of const traits (and `feature(effects)`) from stdlib

The current uses are already unsound because they are using non-const impls in const contexts. We can reintroduce them by reverting the commit in this PR, after #120639 lands.

Also, make `effects` an incomplete feature.

cc `@rust-lang/project-const-traits`
r? `@compiler-errors`

Author: matthiaskrgr

tests/fail/intrinsic_fallback_is_spec.rs

@@ -1,4 +1,4 @@
-#![feature(rustc_attrs, effects)]
+#![feature(rustc_attrs)]
 
 #[rustc_intrinsic]
 #[rustc_nounwind]