sessions: 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: Iffc75a7cb68f22d67548e5632d7ebfbdd67d6598
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>