Make max tip age an option instead of chainparam

After discussion in #7164 I think this is better.

Max tip age was introduced in #5987 to make it possible to run
testnet-in-a-box. But associating this behavior with the testnet chain
is wrong conceptually, as it is not needed in normal usage.
Should aim to make testnet test the software as-is.

Replace it with a (debug) option `-maxtipage`, which can be
specified only in the specific case.

src/chainparams.h

@@ -68,7 +68,6 @@ public:
     bool DefaultConsistencyChecks() const { return fDefaultConsistencyChecks; }
     /** Policy: Filter transactions that do not match well-defined patterns */
     bool RequireStandard() const { return fRequireStandard; }
-    int64_t MaxTipAge() const { return nMaxTipAge; }
     int64_t PruneAfterHeight() const { return nPruneAfterHeight; }
     unsigned int EquihashN() const { return nEquihashN; }
     unsigned int EquihashK() const { return nEquihashK; }
@@ -97,7 +96,6 @@ protected:
     //! Raw pub key bytes for the broadcast alert signing key.
     std::vector<unsigned char> vAlertPubKey;
     int nDefaultPort = 0;
-    long nMaxTipAge = 0;
     uint64_t nPruneAfterHeight = 0;
     unsigned int nEquihashN = 0;
     unsigned int nEquihashK = 0;