[UIFe] Don't show GNOME Books by default

[Jeremy Bicha]

Hi,

I'm requesting a User Interface Freeze Exception to hide the GNOME
Books application by default in Ubuntu 16.04 LTS. Ubuntu GNOME is the
only official Ubuntu flavor to include this app by default.

https://bugs.launchpad.net/bugs/1584522

I don't think there is any Ubuntu-specific documentation for this
feature, nor do I see it mentioned in either the system copy of the
GNOME Documents help or the GNOME help.

I don't think there is any impact on translators either.

Thank you,
Jeremy Bicha