[FIX] mail: channel `#general` with default messages

Before this commit, when starting a new database with demo data,
the default channel `#general` had no messages initially.

The demo data defines default messages in this channel, but were
incorrectly linked to this channel, resulting in this channel
containing no messages initially.

The messages did not define `channel_ids`, so it was considered
empty. This field is essential for displaying the message in
channel, so by being omitted these messages are not properly linked
to channels.

This commit fixes the issue by correctly linked this demo messages
to this demo channel, so that `#general` now contains some messages
initially. This commit also removes unused context
`mail_notify_noemail`.

Task-Id 2186300

closes odoo/odoo#49539

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>