[wpilibc] Clean up integration tests (#3400)

The command and shuffleboard integration tests were removed because
their unit tests counterparts already provide adequate coverage. Java
already removed these.

wpilibcIntegrationTests/src/main/native/dt/Main.cpp

@@ -0,0 +1,5 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// 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.
+
+int main() {}