Remove NSD_ON setting

NSD_ON setting has never been set since it created. The
NsdService is the only user which read/write the setting.
However, NsdManager cts test must running with nsd service
enabled that means this setting should be enabled
always. And this setting is the hidden symbols that
NsdService can't use it after move to mainline module. Thus,
remove this useless setting.

Bug: 206702844
Test: m
Change-Id: Idb5dfc71ebfe2eacd476dd8646aad9adc4975149