Remove redundant explicitly defined copy ctors

Summary:
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 h
ave been generated otherwise.

Backport of core PR11161

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2238