These overloads were needed to allow passing `fs::path` objects directly to
libstdc++'s `fstream` constructors, but after the previous commit, there is no
longer any remaining code that does pass `fs::path` objects to `fstream`
constructors. Writing new code which does this is also discouraged because the
standard has been updated in https://wg21.link/lwg3430 to disallow it.
Dropping these also means its no longer possible to pass `fs::path` arguments
directly to `fstream::open` in libstdc++, which is somewhat unfortunate but not
a big loss because it is already not possible to pass them to the constructor.
So this commit updates `fstream::open` calls.
Additionally, this change required updates to src/bitcoin.cpp since it was
relying on the overloaded filename() method.
c864a4c194