Patch: properly remove accounts from the new gtk list store

[Sergio Villar Senin <svillar igalia com>]

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Hi,

this patch properly removes all the folders of the account that is being
removed from a TnyGtkFolderListStore. The old code was the same as the
one we use for the tree store, and it was not valid, because in the
later removing an account means to remove all the tree of folders,
something that is not true in the former.

Br