Lemma Duplication, `interval_set1` and `set_itv1`

[IshiguroYoshihiro]

Lemmas interval_set1 and set_itv1 have identical statements. https://github.com/math-comp/analysis/blob/eed79156fac45c0aaa5d37aa047e89dbf4481f9c/classical/set_interval.v#L123 https://github.com/math-comp/analysis/blob/eed79156fac45c0aaa5d37aa047e89dbf4481f9c/classical/set_interval.v#L141