[wpimath] Move PIDController from frc2 to frc namespace (#5640)

The old PIDController class in the frc namespace was removed for the
2023 season.

wpimath/src/main/native/include/frc/controller/PIDController.h

@@ -13,7 +13,7 @@
 
 #include "units/time.h"
 
-namespace frc2 {
+namespace frc {
 
 /**
  * Implements a PID control loop.
@@ -279,10 +279,4 @@ class WPILIB_DLLEXPORT PIDController
   bool m_haveMeasurement = false;
 };
 
-}  // namespace frc2
-
-namespace frc {
-
-using frc2::PIDController;
-
 }  // namespace frc