qemuDomainAddCgroupForThread: Don't overwrite the error

Just like in commit 704cf06a, if virCgroup*() fails, the error is
already reported. There's no need to overwrite the error with a
generic one and possibly hiding the true root cause of the error.
Signed-off-by: NLuyao Huang <lhuang@redhat.com>

src/qemu/qemu_driver.c

@@ -4606,9 +4606,6 @@ qemuDomainAddCgroupForThread(virCgroupPtr cgroup,
     /* Add pid/thread to the cgroup */
     rv = virCgroupAddTask(new_cgroup, pid);
     if (rv < 0) {
-        virReportSystemError(-rv,
-                             _("unable to add id %d task %d to cgroup"),
-                             idx, pid);
         virCgroupRemove(new_cgroup);
         goto error;
     }