Remove CGAL_USE_CORE?

[lrineau]

The requirements for CGAL Core have been loosen, and now it is always available.

• Should we remove the occurrences of of the C++ macro CGAL_USE_CORE?
• What about the "component" Core in find_package(CGAL COMPONENTS Core)?

[afabri]

When we remove it in find_package it should not break user CMakeLists,txt files but either be ignored or a message issued that it is no longer needed.