Investigate performance of Flights model after fixing issue #2

[alex-lew]

Performance of Flights model suffers without the subproblem block at

https://github.com/probcomp/PClean/commit/f51c9489dda76a6dbfd7c64fc166a5c94b13db7a#diff-2a3b7234fcda10bae8f2e3e677e2add7dc29ea841a266f8a13708c4e57ac069bR14

but it is unclear to me why this should be the case: the flight ID is always observed.