Remove superfluous maven-antrun-plugin

Remove the maven-antrun-plugin from the pluginManagement section of
spring-boot-starter-parent since it is already declared in the
spring-boot-dependencies POM.

Closes gh-1688

spring-boot-starters/spring-boot-starter-parent/pom.xml

@@ -96,10 +96,6 @@
 						</archive>
 					</configuration>
 				</plugin>
-				<plugin>
-					<groupId>org.apache.maven.plugins</groupId>
-					<artifactId>maven-antrun-plugin</artifactId>
-				</plugin>
 				<plugin>
 					<groupId>org.codehaus.mojo</groupId>
 					<artifactId>exec-maven-plugin</artifactId>