Make subtracting_fractions less computationally intensive

See https://docs.google.com/a/khanacademy.org/document/d/1Ff0WTSkoxxFhQ_6BWcZpEKHSccMT6uUz_m-qVG0CzJU/edit?disco=AAAAAGVNsIw#

Test plan: load a few problems in local mode

Auditors: alpert