Why would you need an op amp for reference voltage when the voltage divider does the trick?
Is there a reason this schematic uses an op amp (AD8031) for the reference voltage of the op amp (AD8544) filter when the voltage divider already brings down the voltage anyways?


The usual reason to use an op-amp to buffer a divider like this is to ensure the reference voltage doesn't change if whatever it's connected to sinks or sources current.


  Can the AD8031 op amp be removed?


In this case, since the AD8544 has only 4 pA input bias current, I'd expect the AD8031 can be removed without much change in performance.

Another issue to watch for, since this reference is connected to two different signals, is whether removing the buffer could allow the two signals to crosstalk with each other. Given the high resistor values connecting the two op-amp inputs to the reference, it's unlikely this would be a real issue, but to be sure you could simply make two different dividers and use one for each of the filter stages.


  Also, to save power, can the voltage divider be replaced with a buck converter?


Any buck converter will have some output ripple. If you used it here, that ripple would be coupled directly into your filtered signal. I wouldn't do it just to save something like 150 uA. (You'd also need to find a buck converter design with less than 150 uA quiescent current to make this a positve trade)

If those 150 uA are really important to your application, you might rather find an op-amp with very low quiescent current (the AD8031 has 800 uA, you'd be looking for 10's of uA), replace the AD8031 with that, and increase the resistor values in the divider to 100 kohm or more.

Aside

The AD8031(A) is only rated to drive capacitive loads up to 15 pF and maintain stability. C2 and C4 in your schematic are probably causing the op-amp to generate noise (it may even be oscillating) rather than reducing noise. I'd remove them.