Remove unused code in src/feedback/EAGLE-kinetic

This PR removes the ability to choose a neighbor selection scheme in EAGLE_kinetic because the feedback in EAGLE_kinetic is always isotropic and the variable feedback_model defined in the file with feedback properties is never used.

I am not sure whether the appearance of feedback_model in EAGLE_kinetic was a mistake during copy-paste of the code from EAGLE_thermal. Or whether the intention was to make the interface in EAGLE_thermal and EAGLE_kinetic as close as possible, and since feedback_model is used EAGLE_thermal, it was copied over to EAGLE_kinetic on purpose.

In any case, I think it is confusing to keep something that is never used, hence I suggest removing feedback_model from EAGLE_kinetic.