Bluetooth: l2cap: Remove (net/buf) frag support

It's needless complexity, and the terminology clashes with
Bluetooth (HCI frags).

It has one user, IPSP, that is going away soon.

Removing frag support will allow a future optimization, removing the
need for HCI and L2CAP fragment buffer pools, saving memory.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>