OPENDJ-1308 Migrate schema support

Replaced getDefinition() by toString() to match SDK types.
Changed toString() to return the definition instead.


SchemaFileElement.java:
Removed getDefinition().

AttributeType.java, CommonSchemaElements.java, DITContentRule.java, DITStructureRule.java, LDAPSyntaxDescription.java, MatchingRuleUse.java, NameForm.java, ObjectClass.java:
In toString(), returned definition.
Removed toStringContent().


Schema.java:
Called toString() instead of getDefinition().
Extracted methods createAttrValueForRemove(), normalizeAttrValue(), createAttrValueForAdd().

SchemaBackend.java:
Called toString() instead of getDefinition().
Extracted deleteFiles() method.

DeleteSchemaElementsTask.java
Called toString() instead of getDefinition().


TestAttributeType.java, TestObjectClass.java:
Removed tests that are no longer testing production code (they are actually testing TestAttributeType.AttributeTypeBuilder).