Remove non-functional endpoints

[joe-crawford]

We wish to remove some endpoints that are non-functional in core. I can't find these being used in mdm-ui but they are exposed in mdm-resources so could the following please be removed:

• /session/keepAlive endpoint (in src/mdm-session.resource.ts) (core issue: 500 error on calling Keep Alive endpoint mdm-core#259)
• dataClasses/clean and dataTypes/clean endpoints (in src/mdm-data-model.resource.ts) (core issue: Permissions error on delete unused data classes and unused data types endpoints on data model mdm-core#272)