It's a Rough Lemon, commonly used as an rootstock by citrus growers. They use it because trees grafted onto it grow faster and bear sooner than with other rootstocks, but if a sucker is allowed to grow, it can rapidly take over the entire tree, killing the citrus you want. If any of the branches are still bearing the little round calamondins, trace them down to the bottom of the tree, and mark them to prevent accidents, then cut everything else off as close to the trunk as you can. If your summers are very hot, you might need to then paint all of the exposed bark with flat, white, interior latex paint to prevent sunburn. As long as you keep the inevitable suckers cleared off on a regular basis--ideally several times a year--the remaining branches will eventually grow into a tree bearing the calamondinsyou wanted. If none of the calamondin part is left alive, you can re-graft the tree (somewhat difficult), discard and replace it, or use the fruit as you would regular lemons--they're just not as juicy.