Replace deprecated Boost Filesystem function (Hennadii Stepanov)
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**.
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Backport of Core [[https://github.com/bitcoin/bitcoin/pull/15880 | PR15880]]