Drop javadoc legacy mode

Due to apache/maven-javadoc-plugin#1217 legacy
mode is no longer compatible with how sourcepath is defined.

Author: akurtakov

eclipse-platform-parent/pom.xml

@@ -639,7 +639,6 @@
             <version>3.11.3</version>
             <configuration>
                 <source>21</source>
-                <legacyMode>true</legacyMode>
                 <doclint>reference,html,syntax</doclint>
                 <splitindex>true</splitindex>
                 <breakiterator>true</breakiterator>