You removed the parent, so you lost its declaration of the WAR plugin configuration. Here it is:
<plugin>
<artifactId>maven-war-plugin</artifactId>
<configuration>
<failOnMissingWebXml>false</failOnMissingWebXml>
</configuration>
</plugin>
See here for the source code.
N.B. this is not necessary with Spring Boot 2.0 parent pom and above (the war plugin version is different), or if you use the latest war plugin.