Rajdhani Unnayan Kartripakkha yesterday started knocking down the much-talked-about BGMEA Bhaban, which was constructed illegally over a decade ago.    Rajuk officials said the 16-storey building is partially blocking Hatirjheel’s water flow in the capital and that it would be demolished in six months.    The building is also like a “tumour on the beauty of Hatirjheel”,