Remove AvroIO deprecated API

[kennknowles]

Imported from Jira BEAM-7208. Original Jira may contain additional context.
Reported by: iemejia.

[Abacn]

This Issue is marked as Beam 3 - Milestone 1: No Code and ML focus. Please help us burning down the open blocker tasks.

• If this issue is solved, close it. If it is obsolete, close as not planned. If it is no longer necessary for Beam 3, move it out of the Milestone.

• If the work is pending, please be reminded that Milestone 1 is targeted for completion by Dec 30, 2025

• If this issue involves multiple milestones (e.g. remove a public API should first mark it as deprecated, scoped in Milestone 1; then finally remove it in Beam 3 release candidate, scoped in Milestone 3) and Milestone 1 portion has completed, consider move the Issue to Milestone 3.

[Abacn]

This sounds a proposed breaking change ideally making the switch between Beam 2->3. Move to milestone 3 for now.