remove deprecated validation endpoints

[karenetheridge]

drop redundant validation_state.validation_plan_id affects endpoints:

• POST /device_report
• POST /device/:id_or_serial_number/validation_plan/:validation_plan_id
• POST /device/:id_or_serial_number/validation/:validation_id
• GET /device/:id_or_serial_number/validation_state

remove deprecated /validation endpoints:

• GET /validation
• GET /validation/:id