[wpilib] Clean up simulation physics API (#2739)

Some vestigial functions were never removed, and C++ single-jointed arm
sim was missing a flag for disabling gravity simulation. This is useful
for mechanisms like turrets.

Fixes #2738.

wpilibc/src/test/native/cpp/simulation/SingleJointedArmSimTest.cpp

@@ -12,7 +12,8 @@
 
 TEST(SingleJointedArmTest, Disabled) {
   frc::sim::SingleJointedArmSim sim(frc::DCMotor::Vex775Pro(2), 100, 10_kg,
-                                    9.5_in, -180_deg, 0_deg, false, {0.0});
+                                    9.5_in, -180_deg, 0_deg, false, {0.0},
+                                    true);
   sim.ResetState(frc::MakeMatrix<2, 1>(0.0, 0.0));
 
   for (size_t i = 0; i < 12 / 0.02; ++i) {