CMakeLists: Revert ad55faaa

The premise behind ad55faaa was due to an issue between Conan's
libiconv package and compiling SDL2 from our externals. Since none of
our Conan externals require libiconv any longer, though, we can remove
downloading our own Boost package and just rely on Conan again.

Additionally, removing CONFIG from the find_package(boost) call fixes
issues with finding Boost on Fedora and MSYS2, which was the main
motivation for this.

Also, remove QUIET since if something goes wrong finding Boost, this
makes it harder to tell what went wrong.