filesystem: use std for DeleteFolder

Remove custom platform implementation of `DeleteFolder` and replace two
uses with `std::filesystem::remove_all` which removes a file or
directory with all its contents.

src/xenia/base/filesystem_posix.cc

@@ -112,12 +112,6 @@ static int removeCallback(const char* fpath, const struct stat* sb,
   return rv;
 }
 
-bool DeleteFolder(const std::filesystem::path& path) {
-  return nftw(path.c_str(), removeCallback, 64, FTW_DEPTH | FTW_PHYS) == 0
-             ? true
-             : false;
-}
-
 static uint64_t convertUnixtimeToWinFiletime(time_t unixtime) {
   // Linux uses number of seconds since 1/1/1970, and Windows uses
   // number of nanoseconds since 1/1/1601