Bluetooth: Remove useless l2cap_seq_list_remove function

The only user of l2cap_seq_list_remove() was l2cap_seq_list_pop() which
only removes the head, meaning only the "else if (seq_list->head ==
seq)" branch was ever being used. This patch moves the code from this
branch straight into l2cap_seq_list_pop() and removes the (now useless)
l2cap_seq_list_remove().

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>