xenapi: Remove vestigial `compile_metrics` code

`compile_metrics` is no longer called as it was superceded by:

    https://github.com/openstack/nova/commit/8887f10c66bca248f289db8f834ae8f36f9a03a1

Fixes bug 1194990

Change-Id: Ia651ac69fc38ed3f5cb29562556f7065caba225a
Merge "xenapi: Remove vestigial `compile_metrics` code"