This commit reverts the namespace validation added in #19696.
The new namespace validation is a breaking change that resulted in users
being locked if roles contained invalid namespaces.
Before this commit, only a single transformation like `email.local` or
`regexp.replace` was allowed in role templates.
With this commit now users can write:
- `email.local(regexp.replace(external.email, "_", "-"))`
- `regexp.replace(email.local(regexp.replace(external.email, "_", "-")), "dev", "admin")`
This commit also allows the above expressions to not contain any
variable, i.e., it allows constant expressions like
`email.local("vitor@goteleport.com")`.