Remove code_user, code_admin roles and remove .code* index privileges for kibana_system role

[kobelb]

Since the experimental code application has now been removed from Kibana, we can remove or modify some of the existing reserved roles as well.

The code_user and code_admin roles can be removed entirely.

Additionally, the kibana_system role can be modified to remove the index-privileges for the code-* and code_internal-* indices.

[elasticmachine]

Pinging @elastic/es-security (:Security/Authorization)

[tvernum]

Thanks for raising this @kobelb.
It has been on my TODO list, but I didn't get around to opening an issue yet.