[documentation] drop capabilities related prlctl and vzctl chapters from documentation

Related description has been already removed from vzctl/prlctl man pages and usage messages (PSBM-41030), need to drop capabilities configuration description from documentation as well (both for Virtuozzo and OpenVZ).

https://docs.openvz.org/openvz_users_guide.webhelp/_linux_specific_capabilities.html
https://docs.openvz.org/openvz_users_guide.webhelp/_available_capabilities_for_containers.html

http://docs.virtuozzo.com/virtuozzo_7_users_guide/advanced-tasks/configuring-capabilities.html
(Please check more places if any.)