Only declare logback-classic as a runtime dependency

This means logback-core is managed as a transitive dependency of
logback-classic and the compatible version is resolved through
the dependency mechanism rather than being manually managed.

Author: sbesson

pom.xml

@@ -166,13 +166,6 @@
     <!-- NB: dependency:analyze has false warning about xml-apis:xml-apis. -->
 
     <dependency>
-      <groupId>ch.qos.logback</groupId>
-      <artifactId>logback-core</artifactId>
-      <version>1.3.14</version>
-      <optional>true</optional>
-    </dependency>
-    <dependency>
-      <!-- NB: We want this, despite warning from dependency:analyze. -->
       <groupId>ch.qos.logback</groupId>
       <artifactId>logback-classic</artifactId>
       <version>1.3.14</version>