PCL is an unused dependency?

[philippremy]

I just noticed that the PCL dependency in Dependencies.cmake is currently unused. No target depends on it and afaik no source file actually uses it. I think it got merged with #1258. But that PR expected a later one to be merged as well (#425), which was actually just closed.

Are there any plans on keeping PCL around or should we remove it as a dead dependency, just like we did for OpenGV (#1980)?