Bug 14066: Remove the Readonly[::XS] dependency

Bug 14066: Remove the Readonly[::XS] dependency

The Readonly and Readonly::XS package are only used in Koha::Calendar.
IMO the added value of there packages does not justify their purpose.

Test plan:
1/ Remove Readonly and Readonly::XS
2/ Open the calendar and confirm you don't get any error concerning missing
dependency

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>