org: remove synth.sh options with missing yosys scripts

Options 4-7 referenced yosys scripts that are no longer in the repo:
nem_synth.ys, bbdd_sifting.ys, no_flatten.ys and sat.ys. Drop the menu
entries, their script variables, the case branches and the compare_blif
function (sole user of sat.ys), and correct the prompt range to [1-3].

Also drop the unused DATA_DIR variable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012nXGcHc3GSLAqg1AL53LKM