global units cannot be unloaded

It seems that `fleetctl unload` does not work on global units:

```
core@core-01 ~/fleet $ ./bin/fleetctl submit examples/foo.service

core@core-01 ~/fleet $ ./bin/fleetctl list-unit-files
UNIT		HASH	DSTATE		STATE	TMACHINE
foo.service	46171e5	inactive	-	-
core@core-01 ~/fleet $ ./bin/fleetctl list-units
UNIT	MACHINE	ACTIVE	SUB

core@core-01 ~/fleet $ ./bin/fleetctl load foo.service
Triggered global unit foo.service load

core@core-01 ~/fleet $ ./bin/fleetctl list-unit-files
UNIT		HASH	DSTATE	STATE	TMACHINE
foo.service	46171e5	loaded	-	-
core@core-01 ~/fleet $ ./bin/fleetctl list-units
UNIT		MACHINE				ACTIVE		SUB
foo.service	29793c23.../172.17.8.101	inactive	dead
foo.service	d4e72abf.../172.17.8.103	inactive	dead
foo.service	eac72a18.../172.17.8.102	inactive	dead

core@core-01 ~/fleet $ ./bin/fleetctl --debug unload foo.service
...
INFO log.go:64: Unit(foo.service) already inactive, skipping.

core@core-01 ~/fleet $ ./bin/fleetctl list-unit-files
UNIT		HASH	DSTATE	STATE	TMACHINE
foo.service	46171e5	loaded	-	-
core@core-01 ~/fleet $ ./bin/fleetctl list-units
UNIT		MACHINE				ACTIVE		SUB
foo.service	29793c23.../172.17.8.101	inactive	dead
foo.service	d4e72abf.../172.17.8.103	inactive	dead
foo.service	eac72a18.../172.17.8.102	inactive	dead
```