If we imagine for a moment that the two current sources are removed, that is, opend, then we shall have two less meshes. But we already have two equations so there will still be enough meshes left for the required number of equations. Moreover, the loops left(they may not be meshes) will have only resistors and voltage sources in them, and therefore KVL is easily applied. We must stress that we are not taking the current sources out. We are only imagining them out for a moment in order to locate the loops around which KVL is to be applied.