[REM] base: broken method Partner._email_send

The underlying `odoo.tools.email_send` was removed in
82de620 (merged in 14.5) but this
callsite was missed, this method has been broken ever since.

I really want to remove this method directly, but technically it's an
expensive no-op if called on a recordset of partners without emails
set (or an empty recordset), so instead make it trigger a warning &
remove in master.

closes odoo#159177

X-original-commit: 457a4be
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>

odoo/addons/base/models/res_partner.py

@@ -8,6 +8,7 @@
 import pytz
 import threading
 import re
+import warnings
 
 import requests
 from collections import defaultdict
@@ -924,8 +925,7 @@ def _get_gravatar_image(self, email):
         return base64.b64encode(res.content)
 
     def _email_send(self, email_from, subject, body, on_error=None):
-        for partner in self.filtered('email'):
-            tools.email_send(email_from, [partner.email], subject, body, on_error)
+        warnings.warn("Partner._email_send has not done anything but raise errors since 15.0", stacklevel=2, category=DeprecationWarning)
         return True
 
     def address_get(self, adr_pref=None):