meta-google: Set entiy-map to `[]` by default for gBMC

The entity map is now used for ipmid/dbus-sdr and will cause ipmi
failures after
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-host-ipmid/+/46882



Remove the default json and only allow system that require it to
override it.

Tested:
```
$ cat ./tmp/work/all-openbmc-linux/phosphor-ipmi-config/1.0-r1/package/usr/share/ipmi-providers/entity-map.json
[]
```

Change-Id: I0ef8248a0d7d99fa0c9e3199c4eceb664bc624cf
Signed-off-by: Willy Tu <wltu@google.com>