Bluetooth: L2CAP: remove metadata allocation

The only thing we put in there is the CID and that fits comfortably into
the (at least) 4-byte void pointer `user_data`.

This removes the dependency between `CONFIG_BT_ATT_TX_COUNT` and
`CONFIG_BT_L2CAP_TX_BUF_COUNT` since previously there was still a need
for an L2CAP context for every TX'd buffer.

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