Remove redundant explicitly defined copy ctors

CFeeRate and CTxMemPoolEntry have explicitly defined copy ctors which has
the same functionality as the implicit default copy ctors which would
have been generated otherwise.

Besides being redundant, it violates the rule of three
(see https://en.wikipedia.org/wiki/Rule_of_three_(C%2B%2B_programming) ).
(Of course, the rule of three doesn't -really- cause a resource
management issue here, but the reason for that is exactly that there is
no need for an explicit copy ctor in the first place since no resources
are being managed).

CFeeRate has an explicitly defined copy ctor which has the same
functionality as the implicit default copy ctor which would have been
generated otherwise.

-----------------------------

Cherry-picked from: b426e2467

Note:
Solved conflict where upstream moved CFeeRate into ./policy/
whereas we still have it in amount.h/cpp on 1.14.

src/amount.h

@@ -44,7 +44,6 @@ public:
     explicit CFeeRate(const CAmount& _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) { }
     /** Constructor for a fee rate in satoshis per kB. The size in bytes must not exceed (2^63 - 1)*/
     CFeeRate(const CAmount& nFeePaid, size_t nBytes);
-    CFeeRate(const CFeeRate& other) { nSatoshisPerK = other.nSatoshisPerK; }
     /**
      * Return the wallet fee in koinus for the given size in bytes.
      */

src/txmempool.cpp

@@ -45,11 +45,6 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransactionRef& _tx, const CAmount& _nFe
     nSigOpCostWithAncestors = sigOpCost;
 }
 
-CTxMemPoolEntry::CTxMemPoolEntry(const CTxMemPoolEntry& other)
-{
-    *this = other;
-}
-
 double
 CTxMemPoolEntry::GetPriority(unsigned int currentHeight) const
 {

src/txmempool.h

@@ -117,8 +117,6 @@ public:
                     CAmount _inChainInputValue, bool spendsCoinbase,
                     int64_t nSigOpsCost, LockPoints lp);
 
-    CTxMemPoolEntry(const CTxMemPoolEntry& other);
-
     const CTransaction& GetTx() const { return *this->tx; }
     CTransactionRef GetSharedTx() const { return this->tx; }
     /**