Merge #15880: utils and libraries: Replace deprecated Boost Filesystem functions

a0a222eec Replace deprecated Boost Filesystem function (Hennadii Stepanov)
4f65af97b Remove dead code for walletFile check (Hennadii Stepanov)

Pull request description:

  Boost Filesystem `basename()` and `extension()` functions are [deprecated since v1.36.0](https://www.boost.org/doc/libs/1_36_0/libs/filesystem/doc/reference.html#Convenience-functions).

  See more: https://lists.boost.org/Archives/boost/2010/01/160905.php

  Also this PR prevents further use of deprecated Boost Filesystem functions.
  Ref: https://www.boost.org/doc/libs/1_64_0/libs/filesystem/doc/index.htm#Coding-guidelines

  Note: On my Linux system Boost 1.65.1 header `/usr/include/boost/filesystem/convenience.hpp` contains:
  ```c++
  # ifndef BOOST_FILESYSTEM_NO_DEPRECATED

      inline std::string extension(const path & p)
      {
        return p.extension().string();
      }

      inline std::string basename(const path & p)
      {
        return p.stem().string();
      }

      inline path change_extension( const path & p, const path & new_extension )
      {
        path new_p( p );
        new_p.replace_extension( new_extension );
        return new_p;
      }

  # endif
  ```

  UPDATE:
  Also removed unused code as [noted](https://github.com/bitcoin/bitcoin/pull/15880#discussion_r279386614) by **ryanofsky**.

ACKs for commit a0a222:
  Empact:
    utACK https://github.com/bitcoin/bitcoin/pull/15880/commits/a0a222eec0b7f615a756e5e0dcec9b02296f999c
  practicalswift:
    utACK a0a222eec0b7f615a756e5e0dcec9b02296f999c
  fanquake:
    utACK a0a222e
  ryanofsky:
    utACK a0a222eec0b7f615a756e5e0dcec9b02296f999c. Only change is dropping assert and squashing first two commits.

Tree-SHA512: bc54355441c49957507eb8d3a5782b92d65674504d69779bc16b1b997b2e7424d5665eb6bfb6e10b430a6cacd2aca70af2f94e5f7f10bea24624202834ad35c7

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