Fix building Javadoc JARs on JDK for client JARs (#29274)

When a module or plugin register that it has a client JAR, we copy
artifacts like the Javadoc and sources JARs as the JARs for the client
as well (with -client added to the name). I previously had to disable
the Javadoc task on JDK 10 due to a bug in bin/javadoc. After JDK 10
went GA without a fix for this bug, I added workaround to fix the
Javadoc task on JDK 10. However, I made a mistake reverting the
previously skipped Javadocs tasks and missed that one that copies the
Javadoc JAR for client JARs. This commit fixes that issue.

buildSrc/src/main/groovy/org/elasticsearch/gradle/plugin/PluginBuildPlugin.groovy

@@ -168,13 +168,11 @@ public class PluginBuildPlugin extends BuildPlugin {
             Files.copy(jarFile.resolveSibling(sourcesFileName), jarFile.resolveSibling(clientSourcesFileName),
                     StandardCopyOption.REPLACE_EXISTING)
 
-            if (project.compilerJavaVersion < JavaVersion.VERSION_1_10) {
             String javadocFileName = jarFile.fileName.toString().replace('.jar', '-javadoc.jar')
             String clientJavadocFileName = clientFileName.replace('.jar', '-javadoc.jar')
             Files.copy(jarFile.resolveSibling(javadocFileName), jarFile.resolveSibling(clientJavadocFileName),
                     StandardCopyOption.REPLACE_EXISTING)
         }
-        }
         project.assemble.dependsOn(clientJar)
     }