xenapi: remove propagate xenapi_use_agent key

The compute.api class will already propagate then xenapi_use_agent key
when required, so we can remove the xenapi specific code that tries to
propagate this value. The code did not correctly access the system
metadata, would have failed to propagate the key anyway.

Fixes bug 1204933
Change-Id: Ib291a265318d80705129e565155d4250fd704b75
Merge "xenapi: remove propagate xenapi_use_agent key"