Remove unnecessary line from Projection::get_z_far

Removes a line from Projection::get_z_far that flips the normal of a plane.
While this may be required for similar code elsewhere in the file, this is
unnecessary here, as only the length of the normal is used and not the
direction. Flipping the normal does not change its magnitude and therefore is
unnecessary in this case.

core/math/projection.cpp

@@ -408,7 +408,6 @@ real_t Projection::get_z_far() const {
 			matrix[11] - matrix[10],
 			matrix[15] - matrix[14]);
 
-	new_plane.normal = -new_plane.normal;
 	new_plane.normalize();
 
 	return new_plane.d;