Bug 2776 : emailPurchaseSuggestions syspref deprecated

Bug 2776 : emailPurchaseSuggestions syspref deprecated

This patch removes the system preference from new installations.
No db revision is supplied to remove the system pref from existing installations.
It is recommended for administrators to use the scripts provided in the misc/ directory
to ensure that their system preferences are in sync with those used in code.
Admins should remove this preference from existing installations manually.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>