build: do not declare javadoc plugin version (#2272)

Author: alicejli

java-common-protos/pom.xml

@@ -146,7 +146,6 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
-        <version>3.6.0</version>
         <reportSets>
           <reportSet>
             <id>html</id>

java-iam/pom.xml

@@ -183,7 +183,6 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-javadoc-plugin</artifactId>
-        <version>3.6.0</version>
         <reportSets>
           <reportSet>
             <id>html</id>