Fix synset_from_sense_key() (#2442)

[ekaf]

Fix #2442, #2420, #2219 and #2171 by reverting to the synset_from_sense_key() implementation from #2621.

from nltk.corpus import wordnet as wn
print(wn.synset_from_sense_key("first%5:00:00:ordinal:00"))

Synset('first.s.02')

Explanation: PR #2621 originally fixed sense_key-related problems (#2420, #2171, plus #2219 and #2442).
But #2889 has inadvertently introduced a non-functional version of synset_from_sense_key(). This problem could have been discovered earlier by running the tests embedded in the documentation string of synset_from_sense_key().

[stevenbird]

Thanks @ekaf