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>

common/params.cc

@@ -251,7 +251,6 @@ std::unordered_map<std::string, uint32_t> keys = {
     {"CarMake", PERSISTENT},
     {"CarModel", PERSISTENT},
 
-    {"CarCruiseDisplayActual", PERSISTENT},
     {"CarSpeedLimit", PERSISTENT},
     
     {"CarSpeedLimitWarning", PERSISTENT},