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;