chore(data/finset): remove duplicated lemma (#14607)
The lemma `ssubset_iff_exists_insert_subset` was added in #11248 but is just a duplicate of the `ssubset_iff` lemma a few lines earlier in the file. It's only used once.
Co-authored-by: Bhavik Mehta <bm489@cam.ac.uk>
