Remove fallbacks for boost_filesystem < v3

Minimum boost version was bumped to 1.47.0 in #8920, which
means the configure step won't even pass with older boost.

This version has boost filesystem v3, which means the
(crappy) fallbacks for older versions can go.

src/wallet/wallet.cpp

@@ -3874,11 +3874,7 @@ bool CWallet::BackupWallet(const std::string& strDest)
                     pathDest /= strWalletFile;
 
                 try {
-#if BOOST_VERSION >= 104000
                     boost::filesystem::copy_file(pathSrc, pathDest, boost::filesystem::copy_option::overwrite_if_exists);
-#else
-                    boost::filesystem::copy_file(pathSrc, pathDest);
-#endif
                     LogPrintf("copied %s to %s\n", strWalletFile, pathDest.string());
                     return true;
                 } catch (const boost::filesystem::filesystem_error& e) {