message registry: Remove odata.context

Redfish made odata.context optional (1.6.0 of DSP0266) and
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.

This removes the last odata.type.

Tested: Built this commit and the commits below and loaded on a
        Witherspoon. No validator errors.
Change-Id: I4cff07b9196f7e9461ea49bb8bcc706e3945e033
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>