(conan): remove sight-deps support

What does this MR do?

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

How to test it?

Just read (and correct) the doc

Data

No data required

Associated Issues/Merge Requests

• Issues

  • Closes #27 (closed)

• Merge Requests

  • sight!208 (merged)