The following message:

    success resolving '<name>' (in '<domain>'?) after reducing the advertised EDNS UDP packet size to 512 octets

can currently be logged even if the EDNS UDP buffer size advertised in
queries sent to a given server had already been set to 512 octets before
the fetch context was created (e.g. due to the server responding
intermittently).  In other words, this log message may be misleading as
lowering the advertised EDNS UDP buffer size may not be the actual cause
of <name> being successfully resolved.  Remove the log message in
question to prevent confusion.

As this log message is the only existing user of the "reason" field in
struct fetchctx, remove that field as well, along with all the code
related to it.