remove deprecated DELETE call from the umapi client

[Luci2015]

Describe the bug
umapi_client contains logic from the deprecated APIs.
this is intended for the DELETE method present in connection.py
make_call function needs adapted to the supported UM API logic.

Environment (please complete the following information):

• Python version: all
• umapi-client version: 2.18

Additional context
Manage Invites under deprecated APIs