Remove system from the list of boost lib to be found

[sloriot]

Remove it from here: https://github.com/norlab-ulaval/libpointmatcher/blob/c44ce315f293f0a03f36cf083a42e398f895d1a7/libpointmatcherConfig.cmake.in#L12

I think it's a leftover from the boost filesystem removal (after checking boost headers included).

It is an issue because in boost master, system is no longer a library and this causes issues when multiple versions of boost are installed on the system