nova: Fix DBNotAllowed during compute startup

backport: stein, rocky

During startup of nova-compute, we see the following error message:

Error gathering result from cell 00000000-0000-0000-0000-000000000000:
DBNotAllowed: nova-compute

This issue was observed in devstack [1], and fixed [2] by removing
database configuration from the compute service.

This change takes the same approach, removing DB config from nova.conf
in the nova-compute* containers.

[1] https://bugs.launchpad.net/devstack/+bug/1812398
[2] https://opendev.org/openstack/devstack/commit/82537871376afe98a286e1ba424cf192ae60869a

Change-Id: I18c99ff4213ce456868e64eab63a4257910b9b8e
Closes-Bug: #1829705