Revert "Assume slice len is bounded by allocation size"

https://github.com/rust-lang/rust/pull/77023#issuecomment-703987379
suggests that the original PR introduced a significant perf regression.

This reverts commit e44784b875 / #77023.

library/core/src/lib.rs

@@ -82,7 +82,6 @@
 #![feature(const_pin)]
 #![feature(const_fn)]
 #![feature(const_fn_union)]
-#![feature(const_assume)]
 #![cfg_attr(not(bootstrap), feature(const_impl_trait))]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]