rm unnecessary .blur() from CloseBracket::placeCursor

unnecessary since caab952cd9d9daeca1c14671f3f221187a8805de

was originally necessary in af15e8089cf25a5f703ea700b28ba9c8b9150d29
because MathCommand::createBlocks special-cased single-block commands,
assuming the cursor would be placed in them and so they'd be focused,
and so didn't blur the block