The ironic-dbsync command by default load service config files.
Removing the override allows us to use additional paths such as
ironic.conf.d in the future.
Note that we still need the hard-coded path for ironic-inspector-dbsync
because the service config file is installed at a path different from
the default one oslo.config attempts to load (the file name should be
"ironic-inspector.conf")
Change-Id: I07e066d240dbe4da3eb2fce9040685cc15997ca0