bluetooth: host: gatt: Compile out att cmds for long write proc

`bt_att_recv`, which parses ATT PDUs, already contains code that
responds with `BT_ATT_ERR_NOT_SUPPORTED` if a given handler is not
implemented.

Therefore, `att_prepare_write_req` and `att_exec_write_req` handlers do
not need to explicitly return `BT_ATT_ERR_NOT_SUPPORTED` when
`CONFIG_BT_ATT_PREPARE_COUNT` is set to 0 and can be completely removed.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>