mathlib

bada932c - feat(data/multiset/basic): `erase_singleton` (#14094)

feat(data/multiset/basic): `erase_singleton` (#14094) Add `multiset.erase_singleton` which is analogous to the existing [finset.erase_singleton](https://leanprover-community.github.io/mathlib_docs/data/finset/basic.html#finset.erase_singleton).