bluetooth: mesh: Remove assertion for Received List message PDU size

According to MshDFUv1.0, section 6.2.3.5, the Firmware Distribution
Receivers List message is not limited by the size of the receivers list:

```
The number of selected entries shall be limited by the following:
- The number of entries shall not exceed the value of the Entries Limit
  field from the Firmware Distribution Receivers Get message.
- The number of entries shall not cause the message payload to exceed
  the maximum Access PDU size.
```

Thus, this assertion is incorrect as it doesn't allow to have the
receivers list bigger than number of maximal Access PDU size.

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