Remove unused function parameters

These were picked up with pylint:

  pylint -d all -e unused-argument nova/compute/manager.py

Note that this returns additional entries but we can't remove those
because they're either (a) part of the API or (b) actually used by
decorators. To summarize the changes:

- Remove 'context' argument from '_set_instance_obj_error_state'
- Remove 'context' argument from '_retry_reboot'
- Remove 'context' argument from '_get_power_state'
- Remove 'context' argument from '_check_instance_exists'
- Remove 'context' argument from '_update_instance_after_spawn'
- Remove 'dp_name' argument from '_get_bound_arq_resources'
- Remove 'context' argument from '_get_power_off_values'
- Remove 'context' argument from '_power_off_instance'

Change-Id: Ifc8fe3b8cc589d27e9d9f00122dffd7e6ed10b13
Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin