USB hub and slave on same pcb - are serial resistors and capacitors needed?
The 15kOhm pull-down resistors are most definitely required. The slave device has internal pull-up resistors on one or both of the DM/DP lines which are used by the host to detect that a slave has been attached and its type - low, full or high speed. You need these pull-down resistors to ensure the line isn't left floating or this detection won't work properly.
You can omit the termination resistors and capacitors. They have two purposes: the first is to prevent reflections from the end of the lines which would cause fading interference with the signal. However, since you are only running the bus in full speed mode (12Mbps) and the line length (5cm) is much much shorter than the wavelength of the signal and its main harmonics, reflections aren't going to be an issue. The second purpose is as a line filter to reduce the EMI generated by the USB cable. You might need some sort of filter to pass EMC tests, but you should just treat the bus lines as you would any other digital lines you have on your board. Note that most USB drivers are slew rate limited so you are already much better off EMC wise than a GPIO line driving your LCD or whatever.