Use lazily expanding flat term everywhere and clean up class

[mezpusz]

I got around to fixing the lazily expanded flat term variant to literals too, which means I could remove the other variant completely and simplify the code a bit.

Minor but relatively stable improvement (mostly for SAT) for both Discount and Otter over FOL TPTP with one seemingly flipping problem instance in both (maybe @quickbeam123 's more stable measurement is needed here. ;)):

Discount:

run 0 unsat: 9208 (1) sat: 1032 (0) cputime: 57559.88 s instructions: 222882769 Mi memory: 1452257.26 MB
run 1 unsat: 9208 (1) sat: 1042 (10) cputime: 57305.22 s instructions: 222743871 Mi memory: 1461347.47 MB

Otter:

run 0 unsat: 9320 (1) sat: 1021 (0) cputime: 62631.56 s instructions: 222819399 Mi memory: 1684459.84 MB
run 1 unsat: 9320 (1) sat: 1025 (4) cputime: 62627.07 s instructions: 222755867 Mi memory: 1693718.59 MB