[export] Remove serialization of old 16-bit nr_devices

We added a 32-bit version of nr_devices on 11/23/2025
but I kept serializating the 16-bit version until now,
in case there are old deserializers still running.

The forward compatibility window of 30-days has passed
and we can stop serializing nr_devices_short.

PiperOrigin-RevId: 861832454

Author: gnecula

jax/_src/export/serialization.py

@@ -144,7 +144,6 @@ def _serialize_exported(
   ser_flatbuf.ExportedAddOutTree(builder, out_tree)
   ser_flatbuf.ExportedAddOutAvals(builder, out_avals)
   ser_flatbuf.ExportedAddNrDevices(builder, exp.nr_devices)
-  ser_flatbuf.ExportedAddNrDevicesShort(builder, exp.nr_devices)  # For forward compatibility, can remove after January 2026
   ser_flatbuf.ExportedAddInShardings(builder, in_shardings)
   ser_flatbuf.ExportedAddOutShardings(builder, out_shardings)
   ser_flatbuf.ExportedAddPlatforms(builder, platforms)