ZmtEclipsePlugin.groovy: fixed cleanEclipse task failing if launch config not present

src/main/groovy/de/zmt/gradle/ZmtEclipsePlugin.groovy

@@ -29,7 +29,7 @@ class ZmtEclipsePlugin implements Plugin<Project> {
 
         // delete launch configuration
         tasks.cleanEclipse.doLast {
-            Files.delete(launchConfig)
+            Files.deleteIfExists(launchConfig)
         }
     }