Removed configuration for maven-compiler-plugin

The configuration for maven-compiler-plugin contains

```
<encoding>UTF-8</encoding>
```

which is not needed cause the default for maven-compiler-plugin encoding usses the property 

```
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
```

which means simply if you have defined the property you con't need to give explicit configuration for encoding.
