(conan): remove sight-deps support

What does this MR do?

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

How to test it?

Configure and build sight

Data

no required data

Additional tests to run

• Configure/Build on Windows
• Configure/Build on Linux
• Configure/Build on macOS

Associated Issues/Merge Requests

• Issues
  • Closes #291 (closed)
• Associated MR
  • Sight/sight-doc!19
  • Sight/sight-deps!15