backport: finish GetDefaultDataDir extraction

Author: thepastaclaw

src/util/system.h

@@ -58,7 +58,6 @@ bool error(const char* fmt, const Args&... args)
 
 void PrintExceptionContinue(const std::exception_ptr pex, const char* pszExceptionOrigin);
 
-fs::path GetDefaultDataDir();
 // Return true if -datadir option points to a valid directory or is not specified.
 bool CheckDataDirOption();
 fs::path GetConfigFile(const fs::path& configuration_file_path);