chore: drop Channel::Voice stub (out of scope for this PR)

The legacy-row stub for Channel::Voice belongs with the upstream
DropChannelVoice migration cleanup, not with WhatsApp call wiring.
Removing it keeps the PR's surface focused on the call pipeline.

enterprise/app/models/channel/voice.rb

@@ -1,8 +0,0 @@
-# Stub for legacy inboxes whose channel_type is still 'Channel::Voice' after
-# the upstream DropChannelVoice migration moved voice fields onto
-# Channel::TwilioSms. Pointing at the new table lets Rails resolve
-# `belongs_to :channel, polymorphic: true` lookups to nil (no matching row),
-# so jbuilder `.try` chains short-circuit instead of raising.
-class Channel::Voice < ApplicationRecord
-  self.table_name = 'channel_twilio_sms'
-end