Remove CMake dependency from "2d" to "io"

If you try to do an algorithms-only build of PCL (without the "io" and "visualization" module), you will stumble accross this dependency which shouldn't exist in my opinion.
The "2d" module contains only a handful of file which implement morphological filters and does not include any files from the "io" module.

2d/CMakeLists.txt

@@ -1,6 +1,6 @@
 set(SUBSYS_NAME 2d)
 set(SUBSYS_DESC "Point cloud 2d")
-set(SUBSYS_DEPS common io filters)
+set(SUBSYS_DEPS common filters)
 
 set(build TRUE)
 PCL_SUBSYS_OPTION(build "${SUBSYS_NAME}" "${SUBSYS_DESC}" ON)