chore: remove duplicate voice channel migration

- Remove db/migrate/20250426130000_create_channel_voice.rb (older duplicate)
- Keep db/migrate/20250620120000_create_channel_voice.rb from develop

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

db/migrate/20250426130000_create_channel_voice.rb

@@ -1,15 +0,0 @@
-class CreateChannelVoice < ActiveRecord::Migration[7.0]
-  def change
-    create_table :channel_voice do |t|
-      t.string :phone_number, null: false
-      t.string :provider, null: false, default: 'twilio'
-      t.jsonb :provider_config, null: false
-      t.integer :account_id, null: false
-      t.jsonb :additional_attributes, default: {}
-      
-      t.timestamps
-      
-      t.index :phone_number, unique: true
-    end
-  end
-end