fix(backup): restrict parallel execution to TakeBackup only

Revert DeleteBackupCommand to executeInSequence()=true per @abh1sar's review.

Backup creation is safe to parallelize across VMs because each VM writes to its
own NAS path. Backup deletion is not safe to parallelize the same way: incremental
chains share parent qcow2 files on the NAS, and concurrent deletions of sibling
or descendant nodes can race on the underlying file references and the chain
metadata. Until that's modeled explicitly, keep DeleteBackup serialized.

TakeBackup parallelization stands.

Author: jmsperu

core/src/main/java/org/apache/cloudstack/backup/DeleteBackupCommand.java

@@ -71,6 +71,6 @@ public void setMountOptions(String mountOptions) {
 
     @Override
     public boolean executeInSequence() {
-        return false;
+        return true;
     }
 }