The named_g_defaultdnstap was never used as the dnstap requires
explicit configuration of the output file.

Related scan-build report:

./server.c:3476:14: warning: Value stored to 'dpath' during its initialization is never read
        const char *dpath = named_g_defaultdnstap;
                    ^~~~~   ~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

(cherry picked from commit 6decd145)