core: Fix not removing old schema
core: Fix not removing old schema

This patch fixes an error when an existent 'schema' is updated.
The current implementation was appending the submitted schema instead
of replacing it.

CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source,CESARBR/knot-service-source