Remove sight-deps support

Description

As of 19.0, Conan becomes the official way to build 3rd party libraries. Since it would be a pain to keep maintaining both sight-deps and Conan together, we agreed to skip the normal deprecation phase.

Proposal

• Modify doc to remove the option USE_CONAN in sight and make it the default. Clean all parts of the build system that refer to EXTERNAL_LIBRARIES.