gnu: java: Return #t from all phases and snippets.

* guix/build/java-utils.scm (ant-build-javadoc): Use invoke; return #t.
* guix/build/ant-build-system.scm (generate-jar-indices): Return #t and
remove vestigal plumbing.
* gnu/packages/java.scm (classpath-bootstrap, ant-bootstrap, classpath-devel)
(icedtea-6, icedtea-7, java-plexus-sec-dispatcher, ant/java8, clojure)
(java-classpathx-servletapi, java-swt, java-qdox-1.12, java-hamcrest-core)
(java-plexus-archiver, java-plexus-sec-dispatcher, java-modello-plugins-xml)
(java-asm, java-commons-collections, java-commons-bsf, java-slf4j-api)
(java-slf4j-api, java-slf4j-simple, java-stringtemplate-3)
(java-stringtemplate, antlr3, antlr3-3.3, antlr3-3.1, java-ops4j-base-lang)
(java-ops4j-pax-tinybundles, java-ops4j-pax-exam-core-spi)
(java-fasterxml-jackson-core, java-fasterxml-jackson-databind)
(java-fasterxml-jackson-modules-base-jaxb, java-ecj-3, java-ecj-3.5)
(java-fasterxml-jackson-dataformat-yaml, java-woodstox-core)
(java-fasterxml-jackson-dataformat-xml, java-testng, java-jnacl)
(java-bouncycastle, java-powermock-core, java-powermock-modules-junit4)
(java-jansi-native, java-jansi, java-commons-httpclient, java-commons-vfs)
(java-apache-ivy, java-janino, java-logback-core): Return #t from all phases
and snippets, use invoke where appropriate, and remove vestigial plumbing.

guix/build/java-utils.scm

@@ -31,7 +31,7 @@
 
 (define* (ant-build-javadoc #:key (target "javadoc") (make-flags '())
                             #:allow-other-keys)
-  (zero? (apply system* `("ant" ,target ,@make-flags))))
+  (apply invoke `("ant" ,target ,@make-flags)))
 
 (define* (install-jars jar-directory)
   "Install jar files from JAR-DIRECTORY to the default target directory.  This