Bug 21955: Remove Cache::Memory as fallback for L2 cache

Bug 21955: Remove Cache::Memory as fallback for L2 cache

If Plack is running with several workers we must not use Cache::Memory
as L2 cache.

If a value is set from a worker, it will not be available from other
workers as the Cache::Memory instance is not shared (of course!)

Moreover we now have Koha::Cache::Memory::Lite that does the same job,
so we should not expect performance regressions by removing it.

See also the email sent to koha-devel for more info
http://lists.koha-community.org/pipermail/koha-devel/2018-December/045004.html

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>