identity: Minor clean up
identity: Minor clean up

There is no need to keep the GoaManager pointer as an instance
variable. We can just get it when adding the account. There are too
many manager-like variables flying around. This makes the code more
readable by keeping one of them closely tied to its context.

https://bugzilla.gnome.org/show_bug.cgi?id=766733

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts