Roles: Remove odata.context

Redfish made odata.context optional (1.6.0 of DSP0266, Sept 2018).
Redfish has removed odata.context from example payloads in the
specification (1.7.0 of DSP0266), removed it from the mockups,
and Redfish recommended not using.

Change-Id: Ic267d8c9f1fbbec6087e61edf16e6bffe11c927e
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>