Revert "Apply flatten plugin."

This reverts commit 86ff0e9.

Author: OlgaMaciaszek

pom.xml

@@ -86,10 +86,6 @@
 		            <argLine>--add-opens java.base/java.util=ALL-UNNAMED</argLine>
 		       </configuration>
 		     </plugin>
-			<plugin>
-				<groupId>org.codehaus.mojo</groupId>
-				<artifactId>flatten-maven-plugin</artifactId>
-			</plugin>
 		</plugins>
 		<pluginManagement>
 			<plugins>