1a7ba84 Fix lack of warning of unrecognized section names (Akio Nakamura)

Pull request description:

  In bitcoin#14708, It was introduced that to warn when unrecognized section names are exist in the config file.
  But ```m_config_sections.clear()```  in ```ArgsManager::ReadConfigStream()``` is called every time when reading each configuration file, so it can warn about only last reading file if ```includeconf``` exists.

  This PR fix lack of warning by collecting all section names by moving ```m_config_sections.clear()```  to ```ArgsManager::ReadConfigFiles()``` .
  Also add a test code to confirm this situation.

Tree-SHA512: 26aa0cbe3e4ae2e58cbe73d4492ee5cf465fd4c3e5df2c8ca7e282b627df9e637267af1e3816386b1dc6db2398b31936925ce0e432219fec3a9b3398f01e3e65

947f73c [docs] remove reference to signrawtransaction in the developer docs. (John Newbery)
7b6616b [rpc] Remove deprecated functionality message from validateaddress help (John Newbery)
839c3f7 [rpc] Remove signrawtransaction warning (John Newbery)

Pull request description:

  Removes some deprecated code from the RPCs:

  - signrawtransaction was deprecated in 0.17 and removed in 0.18. A warning message was left in place to advise users to use signrawtransactionwithwallet and signrawtransactionwithkey. That warning can now be removed.
  - validateaddress had some functionality deprecated in 0.17 and removed in 0.18. The help text for that functionality was not removed in 0.18 and can be removed now.

Tree-SHA512: 981678a697954ff2c392752e5a183b4b12c4eb94f55766ee1aa97a70d300668237db8fc5748c2772869d0155ba4a93e38817887b98160ee972a6f6ee94e3f7d9

…s ignored