Remove JIT-workaround for eliminating bound check

https://github.com/dotnet/runtime/pull/62864 taught the JIT to recognize more pattern to eliminate bound checks for array and span indexing. Thus the workarounds like the `uint`-casts aren't needed anymore, and could be undone.

Also cf. https://github.com/dotnet/runtime/pull/64899#discussion_r833378404

category:cq
theme:bounds-checks
skill-level:beginner
cost:small
impact:small