Hard-deletion doesn't work for older schemas

[crtlib]

A lot of our old schemas have the following key format in _schemas topic:
{"subject":"<subject>","version":1,"magic":1,"keytype":"SCHEMA"}

When we've hard-deleted some of these schemas the following tombstones were emitted:
{"keytype":"SCHEMA","subject":"<subject>","version":1,"magic":1} null

Note, that the key is semantically the same, but it's a different string.
Such a hard deletion doesn't work, cause we see these schemas reappear in the schema registry after some time under unknown to us conditions.