After #28333, LegacyDataSPKM doesn't need to use the private or protected members
of DescriptorScriptPubKeyMan class such as AddDescriptorKeyWithDB and
TopUpWithDB. Moreover, these two SPKMs are siblings that inherit from the common
ScriptPubKeyMan.
It seems reasonable to me that they are unfriended so that private members of one are not exposed to another unnecessarily.
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