OPENDJ-3149 Upgrade 3.0.0 -> 4.0.0: Error in logs after upgrading a replication topology

UpgradeTasks.java:
In restoreCsvDelimiterAttributeTypeInConcatenatedSchemaFile(), also add the X-SCHEMA-FILE
when appending "ds-cfg-csv-delimiter-char" attribute type definition to the concatenated schema

SchemaBackend.java:
In removeAttributeType(), restore code that checked whether the attribute type to be removed
will be added back as part of the same change. This code has been lost during OPENDJ-1632.