Docs: Remove create_default examples with freeze

We freeze objects when calling create_default
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59709. We can now
remove the docs examples.

doc/development/testing_guide/best_practices.md

@@ -168,7 +168,7 @@ can be used:
 
 ```ruby
 RSpec.describe API::Search, factory_default: :keep do
-  let_it_be(:namespace) { create_default(:namespace).freeze }
+  let_it_be(:namespace) { create_default(:namespace) }
 ```
 
 Then every project we create uses this `namespace`, without us having to pass
@@ -186,7 +186,7 @@ projects we create are ones we ask for (76/208). There is benefit in setting
 a default value for projects as well:
 
 ```ruby
-  let_it_be(:project) { create_default(:project).freeze }
+  let_it_be(:project) { create_default(:project) }
 ```
 
 In this case, the `total time` and `top-level time` numbers match more closely: