chore: remove code nothing uses

Each was checked with git grep across every tracked file (templates, JS,
shell, unit files and the extensionless console daemons) and for
dynamic use (URL names, template names, registries):

- vrtManager: host_is_up, is_kvm_supported, get_emulators,
  get_file_extensions, both get_network_forward, get_snapshots,
  get_storages_images, get_os_type, get_host_arch, the get_pretty_* trio
  and, in util, is_kvm_available, randomMAC, get_max_vcpus, compareMAC,
  pretty_mem, pretty_bytes and validate_uuid; with them the port
  constants only host_is_up read. Some were reached only by their tests,
  which go too.
- The flavor edit page: nothing linked to it; flavors are created and
  deleted on the Flavors page and edited through the API.
- computes.validate_name, UserForm.identity_has_errors, the has_perm
  template tag, the ALLOW_EMPTY_PASSWORD setting nothing reads,
  hidePleaseWaitDialog, default_app_config (ignored since Django 4.1)
  and the Python < 3.6 lines of secret_generator.py.

admin/forms.py

@@ -119,10 +119,6 @@ class UserForm(UniqueEmailMixin, forms.ModelForm):
     def permission_rows(self):
         return permission_rows(self["user_permissions"])
 
-    def identity_has_errors(self):
-        return any(self.has_error(name) for name in ("username", "password", "first_name", "last_name", "email"))
-
-
 class UserCreateForm(UserForm):
     password = forms.CharField(widget=forms.PasswordInput)