[export] Remove code to serialize HloSharding
This creates the export serialization version 9, removing
from the serialized exports the fields `in_hlo_shardings` and
`out_hlo_shardings`. On 1/15/2026 we have added the fields
`in_named_shardings` and `out_named_shardings` to the serialization
and since the 30-day forward compatibility window has passed
we can assume that all readers of exports will now be able to
operate with the named shardings.
Note that we need to still keep the support for `in_hlo_shardings`
and `out_hlo_shardings` when **deserializing** old exports, until
6 months after 1/15/2026. But at least we can avoid constructing
the HloShardings when creating and serializing exports.
I have added new versions for the export_serialization_back_compat_test.py.
This is another attempt at landing #35557, which had to be rolled back
due to compatibility failures.
