Drop the requirement of no locked platforms in pathways.txt

### Describe the problem

The specifications of `pathways.txt` mentions that,

> No locked platforms: Each platform (location_type=0 or empty) or boarding area (location_type=4) must be connected to at least one entrance/exit (location_type=2) via some chain of pathways. Stations not allowing a pathway to the outside of the station from a given platform are rare.

The fact is that, there exist some stations where access to the outside world is prohibited. These stations are for interchange only. The most famous example in Hong Kong is called Shing Mun Tunnels Bus Interchange, which is an interchange-only station with no public access to outside the station.

### Use cases

We need to define stations which are interchange only, i.e. no access to the outside world.

### Proposed solution

Delete the quoted paragraph. Change the validator to produce a warning instead.

### Additional information

_No response_