redhat_subscription: refactor of internal Rhsm class (#6658)

The two RegistrationBase & Rhsm classes were copied from the ones in the
shared module_utils.redhat module; that said:
- the versions here got improvements over the years
- the RegistrationBase in module_utils.redhat is used only by the RHN
  modules, which are deprecated and slated for removal

Hence, the classes here can be kept and simplified a bit:
- fold the non-dummy content of RegistrationBase into Rhsm: there is no
  more need for the separate RegistrationBase base class
- drop the init arguments "username", "password", and "token": the
  instance variables of them are not used anywhere, as the needed
  credentials (together with other variables) are passed to the
  register() method
- create the Rhsm object later in main(), after the AnsibleModule
  creation and the uid check: this avoids the creation of Rhsm with a
  null module variable, changing it later

There should be no behaviour change.

changelogs/fragments/6658-redhat_subscription-internal-rhsm-refactor.yml

@@ -0,0 +1,6 @@
+minor_changes:
+  - |
+    redhat_subscription - the internal ``RegistrationBase`` class was folded
+    into the other internal ``Rhsm`` class, as the separation had no purpose
+    anymore
+    (https://github.com/ansible-collections/community.general/pull/6658).