[build] remove explicit commons-codec in assembly file

We don't have to explicitly define `commons-codec` in our assembly file.

When removing it, the content is still exactly the same:

```
Archive:  target/releases/elasticsearch-analysis-phonetic-2.4.4-SNAPSHOT.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
    17193  05-07-15 09:08   elasticsearch-analysis-phonetic-2.4.4-SNAPSHOT.jar
    23098  03-17-15 00:28   lucene-analyzers-phonetic-4.10.4.jar
   263965  11-09-14 14:02   commons-codec-1.9.jar
 --------                   -------
   304256                   3 files
```

See https://github.com/elastic/elasticsearch-analysis-phonetic/issues/40

Closes #44.

(cherry picked from commit 6fa3fb8)

Author: dadoonet

src/main/assemblies/plugin.xml

@@ -20,8 +20,7 @@
             <useTransitiveFiltering>true</useTransitiveFiltering>
             <includes>
                 <include>org.apache.lucene:lucene-analyzers-phonetic</include>
-                <include>commons-codec:commons-codec</include>
             </includes>
         </dependencySet>
     </dependencySets>
-</assembly>
+</assembly>