Remove mesos API validation

As discussed in meeting[1], we will drop mesos from the API in this
version, and merge the remainder of the removal code i.e. Change
Ied76095f1f1c57c6af93d1a6094baa6c7cc31c9b in the next version

[1] https://meetings.opendev.org/meetings/magnum/2022/magnum.2022-02-16-09.00.html

Story: 2009873
Task: 44581
Change-Id: If868d117827d6ba0331a89650d5fab986b5592f1

Change-Id: Ie778894815b2dca4ce91661ee4f28c9496a77976

Author: waipeng

magnum/api/validation.py

@@ -256,8 +256,6 @@ def get_coe_validator(cls, coe):
             return K8sValidator()
         elif coe == 'swarm' or coe == 'swarm-mode':
             return SwarmValidator()
-        elif coe == 'mesos':
-            return MesosValidator()
         else:
             raise exception.InvalidParameterValue(
                 _('Requested COE type %s is not supported.') % coe)

releasenotes/notes/disable-mesos-from-api-0087ef02ba0477df.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    Remove mesos from the API. This means new clusters of coe type 'mesos'
+    cannot be created.
+    The mesos driver will be removed in the next release.