Revert disjointness->disjointedness (#14906)
## Summary
Partially revert #14880. While `disjointness` is missing from the
[OED](https://www.oed.com/search/dictionary/?q=disjointness) and [SCOWL
(And
Friends)](http://app.aspell.net/lookup?dict=en_US-large;words=disjointness),
it is commonly used in mathematics to describe disjoint sets.
## Test Plan
CI tests.
