Removal of eigen3 rosdep rule

From an question on I noticed that we’ve ended up with semi overlapping duplicate entries for eigen.

I’ve submitted a PR to propose removing the less complete key eigen3 which only has fedora and gentoo rules. Those same rules exist using the eigen rosdep key.

Thie PR is here: please comment if this will effect you adversely. If you’re using this rule you can simply change to use the eigen key as a dependency instead of eigen3.