Remove or fix function declarations that don't match any definition.
Split out of #36743. Ideally we can have the compiler enforce/flag this in future.
The header declared AddInputs(..., bool rbf), which was never defined,
while the definition takes std::optional<bool> and has no declaration.
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These were declared in fa6b061fc1 but never defined or used.
The definitions were removed in 43c1f5b8d7, but the declarations were
left behind.
Match the definition, and the m_idle_since member.