remove FindEigen cmake script, so the system script is used

The FindEigen.cmake script should not be needed (at least in fuerte and more recent ROS versions), since ROS/catkin already provides one.
If the script in the cmake folder doesn't find eigen (e.g. installed in a non-default location), it will fail, even if the system FindEigen.cmake works correctly.

Tested with rosmake under fuerte and groovy.