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**.

ninja check check-functional