DROP CONSTRAINT ON

Drops a property constraint on a node label or relationship type

The DROP CONSTRAINT ON clause will delete a property constraint

Reference: schema constraints manual page
Related: :help CREATE CONSTRAINT ON :help Schema :help Cypher
DROP CONSTRAINT ON (p:Person)
ASSERT p.name IS UNIQUE
Drop the unique constraint and index on the label Person and property name.
DROP CONSTRAINT ON (p:Person)
ASSERT exists(p.name)
Drop the node property existence constraint on the label Person and property name.
DROP CONSTRAINT ON ()-[l:LIKED]-()
ASSERT exists(l.when)
Drop the relationship property existence constraint on the type LIKED and property when.