Connection.GetObject: do not check that the bus name exists

This check did not exist in 0.7, but was added in e0eed784d7 before 0.8.

When D-Bus service activation is used, the desired bus name is not
going to exist at this point; when we call a method, the dbus-daemon
transparently starts the service before delivering the message.
This is particularly important for services that are not normally
running before they are needed, such as polkit and systemd-hostnamed.
Calling NameHasOwner also doubles the number of method calls if you're
only making one method call on the object, and has a race condition:
if the service exits between the successful NameHasOwner call and the
next method call, the caller will still have to deal with that.

Working around the check by doing a StartServiceByName first is
inefficient, and also introduces a race condition: if the service
exits between StartServiceByName success and NameHasOwner, then
GetObject would still return null.