virtual media: 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.

The reason for making optional and removing from mockups/examples,
"no one could figure out how to use it and it did not add value".

Don't see value in it for our implementation.

Change-Id: I3972fea43d89267ba91f5a0f7c0aac00a18e0f6c
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>