[Routing] simplify the XML schema file

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Conditions that are defined for certain routes are just simple strings.
Thus, there is no need to use a dedicated element when xsd:string does
the same (as mentioned by @tobion in #11394).