remove coverage badge because of incorrect coverage report
remove coverage badge because of incorrect coverage report

Problem is that jacoco execution data for integration tests cannot be
"matched" and the following output is generated:

```
[ant:jacocoReport] Classes in bundle 'taskwarrior-java-client' do no match with execution data. For report generation the same class files must be used as at runtime.
[ant:jacocoReport] Execution data for class de/aaschmid/taskwarrior/client/TaskwarriorClient does not match.
[ant:jacocoReport] Execution data for class de/aaschmid/taskwarrior/config/TaskwarriorConfiguration does not match.
[ant:jacocoReport] Execution data for class de/aaschmid/taskwarrior/message/ManifestHelper does not match.
```

Signed-off-by: Andreas Schmid <4cf5bc59bee9e1c44c6254b5f84e7f066bd8e5fe@aaschmid.de>

aaschmid/taskwarrior-java-client,aaschmid/taskwarrior-java-client