Delete Util::StringPrintf.
Delete Util::StringPrintf.

All the calls were migrated to absl::StrFormat and, hence, it is no longer
necessary.

PiperOrigin-RevId: 409305236

google/mozc,fcitx/mozc,fcitx/mozc,fcitx/mozc,fcitx/mozc,fcitx/mozc,google/mozc,google/mozc,google/mozc,google/mozc