[BLE] Remove old TODO (#14747)
[BLE] Remove old TODO (#14747)

#### Problem

Issue #13939 Optimize Packet buffer usage in CHIPoBLE ACK management

> TODO Packet buffer high water mark optimization: if ack pending,
> but fragmenter state == complete, free fragmenter's tx buf before
> sending ack.

This TODO is seven years old and not worth the risk of implementing
for 1.0.

#### Change overview

Remove the TODO to placate automated scans.
Issue will be left for future consideration.

#### Testing

None; changes comment only.
project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,nestlabs/connectedhomeip,project-chip/connectedhomeip,project-chip/connectedhomeip,nestlabs/connectedhomeip