Remove reduration 'samm:optional true' attribute in property

Author: Yauhenikapl

core/esmf-test-aspect-models/src/main/resources/valid/org.eclipse.esmf.test/1.0.0/AspectWithOptionalPropertiesAndEntityWithSeparateFiles.ttl

@@ -64,7 +64,6 @@
    samm:characteristic :YearCharacteristic .
 
 :month a samm:Property ;
-   samm:optional true ;
    samm:characteristic :MonthCharacteristic .
 
 :YearCharacteristic a samm:Characteristic ;