nbd-server: remove unused function

Something like this might be needed in the future, once dynamic
reconfiguration has landed. But it's better to rewrite and test it then
and not keep unreferenced code hanging around cluttering the code base.

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>
nbd-server: remove commented code

Signed-off-by: Tuomas Jorma Juhani Räsänen <tuomasjjrasanen@tjjr.fi>