matrix: remove wlr_matrix_projection()

69477051cc ("matrix: deprecate wlr_matrix_projection") marked it
as deprecated. 1 year later, we can now remove it from our public
API.

include/types/wlr_matrix.h

@@ -0,0 +1,15 @@
+#ifndef TYPES_WLR_MATRIX_H
+#define TYPES_WLR_MATRIX_H
+
+#include <wlr/types/wlr_matrix.h>
+
+/**
+ * Writes a 2D orthographic projection matrix to mat of (width, height) with a
+ * specified wl_output_transform.
+ *
+ * Equivalent to glOrtho(0, width, 0, height, 1, -1) with the transform applied.
+ */
+void matrix_projection(float mat[static 9], int width, int height,
+	enum wl_output_transform transform);
+
+#endif