Remove non linear controller

src/nonlinear.py

-import numpy as np
-import rospy
-
-from controller import BaseController
-
-
-# Uses Proportional-Differential Control from
-# https://www.a1k0n.net/2018/11/13/fast-line-following.html
-class NonLinearController(BaseController):
-    def __init__(self):
-        super(NonLinearController, self).__init__()
-
-    def get_reference_index(self, pose):
-        assert False, "Complete this function"
-
-    def get_control(self, pose, index):
-        assert False, "Complete this function"
-
-    def reset_state(self):
-        with self.path_lock:
-            assert False, "Complete this function"
-
-    def reset_params(self):
-        with self.path_lock:
-            assert False, "Complete this function"