fix a lemma that did not use its stuckness

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passed #6321
fp-timing
build-coq.8.6.1

00:02:57

passed #6320
fp-timing
build-coq.8.7.0

00:02:49

 
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opam

00:00:14

passed #6323
reverse-deps

00:00:14

 
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passed #6324
fp-timing
build-coq.8.7.dev

00:03:02