Bluetooth: host: Remove BT_LE_CONN_OPT_2M option

Remove the BT_LE_CONN_OPT_2M option and update documentation.
This was a misunderstand about the init PHY HCI parameter.
The init PHY in the extended connection create command does not
determine which PHYs are accepted as the initial PHY of the
connection. This is instead determined by the secondary PHY of the
extended advertiser.
The init PHY parameter only specifies which conn parameters are
provided, and since we only provide one conn parameter this option
has no effect.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>