cleanup: remove dead CarCruiseDisplayActual param

Audit of post-fork param additions found CarCruiseDisplayActual was
written every CAN cycle (gated) but only consumed by hyundaicanfd.py::
create_buttons_alt, which has `return` on line 1 and no active callers.
Write was pure waste. Removed registration, write path, cache field,
and the dead read.

Also dropped the now-unused `from openpilot.common.params import Params`
in hyundaicanfd.py.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/car/hyundai/hyundaicanfd.py

@@ -2,7 +2,6 @@ from openpilot.common.numpy_fast import clip
 from openpilot.selfdrive.car import CanBusBase
 from openpilot.selfdrive.car.hyundai.values import HyundaiFlags
 
-from openpilot.common.params import Params
 
 class CanBus(CanBusBase):
   def __init__(self, CP, hda2=None, fingerprint=None) -> None:
@@ -127,9 +126,7 @@ def create_buttons(packer, CP, CAN, cnt, btn):
 
 def create_buttons_alt(packer, CP, CAN, cnt, btn, template):
   return
-  params_memory = Params("/dev/shm/params")
-  CarCruiseDisplayActual = params_memory.get_float("CarCruiseDisplayActual")
-  
+
   values = {
     "COUNTER": cnt,
     "NEW_SIGNAL_1": 0.0,