He's not getting rid of them now. He'll just prevent any of his Remainer MPs from standing at the next General Election if they try to scupper No Deal Brexit. And he's quite right to do so. The Tories won the election on a mandate to deliver Brexit.
Of course, Boris will just do the voters' job for them. Even if he doesn't prevent any Remainer Tory MPs from standing at the next election for attempting to thwart No Deal Brexit then their constituents would just kick them out anyway and likely vote in a Brexit Party MP instead. So Boris is trying to ensure there'll still be Tories in those seats after the election rather than Brexit Party MPs.
I can't wait until the next General Election - the looks on the faces of all those Remainer MPs of Leave constituencies (and there are a lot of such MPs, the majority, in fact) when they have just discovered they've been kicked out of office by their constituents will be priceless.