Fixes duplicate .xsd files in JAR file

This pull request fixes an issue that was preventing me from including `voldemort-1.3.0.jar` in a Leiningen project. The problem is that the build script includes duplicate copies of the `.xsd` files (`cluster.xsd` and `stores.xsd`) in the `.jar` file. This causes the following exception when trying to include `voldemort-1.3.0.jar` in another `.jar` file:

    java.util.zip.ZipException: duplicate entry: voldemort/xml/cluster.xsd

Before building, the `.xsd` files are in `src`:

    $ ant clean
    $ find . -name '*.xsd'
    ./src/java/voldemort/xml/cluster.xsd
    ./src/java/voldemort/xml/stores.xsd

After building, the `.xsd` files are copied to `dist`:

    $ ant
    $ find . -name '*.xsd'
    ./dist/classes/voldemort/xml/cluster.xsd
    ./dist/classes/voldemort/xml/stores.xsd
    ./src/java/voldemort/xml/cluster.xsd
    ./src/java/voldemort/xml/stores.xsd

The files are the same:

    $ diff src/java/voldemort/xml/cluster.xsd dist/classes/voldemort/xml/cluster.xsd
    $ diff src/java/voldemort/xml/stores.xsd dist/classes/voldemort/xml/stores.xsd

The rule which I removed from `build.xml` was including both copies in the `.jar` file:

    $ jar tf dist/voldemort-1.3.0.jar | grep '\.xsd$'
    voldemort/xml/cluster.xsd
    voldemort/xml/stores.xsd
    voldemort/xml/cluster.xsd
    voldemort/xml/stores.xsd

After building with my modified `build.xml`, only the `.xsd` files from `dist/classes` are included in the `.jar` file:

    $ tar tf dist/voldemort-1.3.0.jar | grep '\.xsd$'
    voldemort/xml/cluster.xsd
    voldemort/xml/stores.xsd