Jenkinsfile: Remove unnecessary cleanall

The previous script was always cleaning the rootfs which is not
required. It build automatically be rebuilt if something has changed or
kept as is if nothing has changed.

Jenkinsfile

@@ -120,7 +120,7 @@ def build(env_in, image_type_in, single_fitImage) {
   if (single_fitImage) {
     sh "bash -c '. ./env.image${env} && bitbake -k virtual/kernel'"
   } else {
-    sh "bash -c '. ./env.image${env} && bitbake -fc cleanall netmodule-linux-image${image_type} && bitbake -k netmodule-linux-image${image_type}'"
+    sh "bash -c '. ./env.image${env} && bitbake -k netmodule-linux-image${image_type}'"
   }
 }