Fix cleanup of Environment class. (#25743)
### Description
<!-- Describe your changes. -->
The clearing of shared_allocators_ invalidates all entries in
shared_ort_allocators_.
Remove unused shared_arena_allocators_. That became unnecessary by
providing EPs an example implementation for an OrtAllocator based
stream-aware arena that they can use directly.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Fix access violation (swallowed as it happens during shutdown) in dtor.
