Cleanup logging dependencies

removed the global test dependency on slf4j-nop, because it clutters the
dependency information and is in many cases just wrong (e.g. the build-tools
module has no such dependency).

As a consequence, the maven tests now log error messages about missing slf4j
implementations again. For now however, this seems to be the best solution,
since none of the LearnLib modules actually require a logging implementation
as it is merely a dependency for the end-user, who will use LearnLib.

The best example is the 'examples' module, for which a logback runtime
dependency was added.

Author: mtf90

examples/pom.xml

@@ -110,5 +110,11 @@ limitations under the License.
             <groupId>net.automatalib</groupId>
             <artifactId>automata-commons-dotutil</artifactId>
         </dependency>
+
+        <dependency>
+            <groupId>ch.qos.logback</groupId>
+            <artifactId>logback-classic</artifactId>
+            <scope>runtime</scope>
+        </dependency>
     </dependencies>
 </project>

pom.xml

@@ -1096,12 +1096,6 @@ limitations under the License.
                 <version>${slf4j.version}</version>
             </dependency>
 
-            <dependency>
-                <groupId>org.slf4j</groupId>
-                <artifactId>slf4j-nop</artifactId>
-                <version>${slf4j.version}</version>
-            </dependency>
-
             <dependency>
                 <groupId>ch.qos.logback</groupId>
                 <artifactId>logback-core</artifactId>
@@ -1117,15 +1111,6 @@ limitations under the License.
         </dependencies>
     </dependencyManagement>
 
-    <dependencies>
-        <!-- prevent error messages about missing slf4j implementation during test executions -->
-        <dependency>
-            <groupId>org.slf4j</groupId>
-            <artifactId>slf4j-nop</artifactId>
-            <scope>test</scope>
-        </dependency>
-    </dependencies>
-
     <pluginRepositories>
         <!-- Ensure preference over the atlassian plugin repository -->
         <pluginRepository>