[FIX] im_livechat: disable attachment upload via drag and drop

Before this commit:

Attachments are not properly supported in livechat, so there is no point in
allowing attachments to be uploaded via drag and drop.

After this commit:

Disable attachment upload via drag and drop in livechat channels.

Task-2824502

closes odoo/odoo#95274

X-original-commit: 61f83a11
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>