Remove open "ignorable public" array types (#6940)

There are a few heap types that are hard-coded to be considered public
and therefore allowed on module boundaries even in --closed-world mode,
specifically to support js-string-builtins. We previously considered
both open and closed (i.e. final) mutable i8 arrays to be public in this
manner, but js-string-builtins only uses the closed versions, so remove
the open versions.

This fixes a particular bug in which Unsubtyping optimized a private
array type to be equivalent to an ignorable public array type,
incorrectly changing the behavior of a cast, but it does not address the
larger problem of optimizations producing types that are equivalent to
public types. Add a TODO about that problem for now.

Fixes #6935.

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