Remove uses of iostream (#4004)

Most of these were unused, the IMU ones were just debug messages.

The only one that wasn't removed is in portable-file-dialogs.cpp since
the replacement looks less trivial.

wpimath/src/test/native/cpp/spline/CubicHermiteSplineTest.cpp

@@ -3,7 +3,6 @@
 // the WPILib BSD license file in the root directory of this project.
 
 #include <chrono>
-#include <iostream>
 #include <vector>
 
 #include "frc/geometry/Pose2d.h"

wpimath/src/test/native/cpp/spline/QuinticHermiteSplineTest.cpp

@@ -3,7 +3,6 @@
 // the WPILib BSD license file in the root directory of this project.
 
 #include <chrono>
-#include <iostream>
 
 #include "frc/geometry/Pose2d.h"
 #include "frc/geometry/Rotation2d.h"

wpimath/src/test/native/cpp/trajectory/DifferentialDriveVoltageTest.cpp

@@ -2,7 +2,6 @@
 // Open Source Software; you can modify and/or share it under the terms of
 // the WPILib BSD license file in the root directory of this project.
 
-#include <iostream>
 #include <memory>
 #include <vector>