`rb_ivar_delete` would force a new shape to be created.
And in the case of a shape exhaustion, it wouldn't handle
T_CLASS/T_MODULE correctly.

Its usage was removed in 306d50811dd060d876d1eb364a0d5e6106f5e4f1.

32-bit platforms do not have flonum and something about the static symbol test was flaky.

It only makes sense for heap objects.

It is much more convenient than storing the klass, especially
when dealing with `object_id` as it allows to update the id2ref
table without having to dereference the owner, which may be
garbage at that point.

If `get_next_shape_internal` fail to return a shape, we must
transitiont to a complex shape. `shape_transition_object_id`
mistakenly didn't.

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

Now that we have the `heap_index` in shape flags we no longer
need `T_OBJECT` shapes.

This is preparation to getting rid of `T_OBJECT` transitions.
By first only replicating the information it's easier to ensure
consistency.

Now all flags are only in the `shape_id_t`, and can all be checked
without needing to dereference a pointer.