Revert "Restrict relative mouse mode to Wayland users (#15697)"

see #15761
SDL is the only device that supports relative mode and
mouse input is actually somewhat broken if it's *not* enabled.

This reverts commit 45c5ef8798
and 88b007907a.

irr/include/IrrlichtDevice.h

@@ -198,9 +198,6 @@ public:
 	or similar. */
 	virtual bool supportsTouchEvents() const { return false; }
 
-	//! Checks whether windowing uses the Wayland protocol.
-	virtual bool isUsingWayland() const { return false; }
-
 	//! Get the current color format of the window
 	/** \return Color format of the window. */
 	virtual video::ECOLOR_FORMAT getColorFormat() const = 0;

irr/src/CIrrDeviceSDL.cpp

@@ -1256,15 +1256,6 @@ bool CIrrDeviceSDL::supportsTouchEvents() const
 	return true;
 }
 
-//! Checks whether windowing uses the Wayland protocol.
-bool CIrrDeviceSDL::isUsingWayland() const
-{
-	if (!Window)
-		return false;
-	auto *name = SDL_GetCurrentVideoDriver();
-	return name && !strcmp(name, "wayland");
-}
-
 //! returns if window is active. if not, nothing need to be drawn
 bool CIrrDeviceSDL::isWindowActive() const
 {

irr/src/CIrrDeviceSDL.h

@@ -96,9 +96,6 @@ public:
 	//! Checks if the Irrlicht device supports touch events.
 	bool supportsTouchEvents() const override;
 
-	//! Checks whether windowing uses the Wayland protocol.
-	bool isUsingWayland() const override;
-
 	//! Get the position of this window on screen
 	core::position2di getWindowPosition() override;