[morzh]

If it was wired across the filter choke, you could take it out.
It was a thought that a cap across the choke could pass just enough ripple to compensate the residual ripple remaining after the LC filter, due to the fact that the currents in the choke and the cap are phased 180 degrees to each other when the voltage is the same, so they would cancel out.

The practice confirmed this is not that efficient and simply increasing the cap after the choke is way more efficient at suppression of the ripple and, therefore, the hum.

The limits for the cap capacitance that the rectifier tube has applies only to the first cap, connected directly to its Cathode, but the cap after the choke could be increased quite a bit as the ripple current across it is fairly small. So while the cap 56 should be 6uF as required (small variations are OK as the tolerance itself is 20%) the cap 57 could be 10uF, 22uF or even more. And the one in question is simply not needed.