Bluetooth: hci_sync: fix double mgmt_pending_free() in remove_adv_monitor()

syzbot is reporting double kfree() at remove_adv_monitor() [1], for
commit 7cf5c297 ("Bluetooth: hci_sync: Refactor remove Adv
Monitor") forgot to remove duplicated mgmt_pending_remove() when
merging "if (err) {" path and "if (!pending) {" path.

Link: https://syzkaller.appspot.com/bug?extid=915a8416bf15895b8e07

 [1]
Reported-by: syzbot <syzbot+915a8416bf15895b8e07@syzkaller.appspotmail.com>
Fixes: 7cf5c297

 ("Bluetooth: hci_sync: Refactor remove Adv Monitor")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>