ospfd: Remove flog_warn for a situation user can never do anything with

ospfd: Remove flog_warn for a situation user can never do anything with

When OSPF receives a Database description packet and is in
`Down`, `Attempt` or `2-Way` state we are creating a warning
for the end user.

rfc2328 states(10.6):
Down - The packet should be rejected
Attempt - The packet should be rejected
2-Way - The packet should be ignored

I cannot find any instructions in the rfc to state what the operational
difference is between rejected and ignored.  Neither can I figure
out what FRR expects the end user to do with this information.
I can see this information being useful if we encounter a bug
down the line and we have gathered a bunch of data.  As such
let's modify the code to remove the flog_warn and convert
the message to a debug level message that can be controlled by
appropriate debug statements.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>