remove inst_servers webserver, remove fallback for JSON_bourne configuration information

As a developer I would like to maintain one less process on the instruments, by removing https://github.com/ISISComputingGroup/EPICS-inst_servers/tree/master/WebServer which is used as a fallback for JSON_Bourne when getting the configuration information (not the actual PV info themselves)

This will involve removing https://github.com/ISISComputingGroup/EPICS-inst_servers/blob/master/block_server.py#L475 and some other lines which use this code as well as the [fallback code](https://github.com/ISISComputingGroup/JSON_bourne/blob/master/external_webpage/data_source_reader.py#L123) in JSON bourne.

### Acceptance criteria
- [ ] webserver is removed from JSON_bourne
- [ ] all references of it are removed from other code

note that this is really because JSON_bourne is going to become a fallback for https://isiscomputinggroup.github.io/WebDashboard/ so it doesn't make sense to keep the web server running on instruments.