util: Replace i64tostr with ToString

src/util/strencodings.cpp

@@ -407,11 +407,6 @@ std::string FormatParagraph(const std::string& in, size_t width, size_t indent)
     return out.str();
 }
 
-std::string i64tostr(int64_t n)
-{
-    return strprintf("%d", n);
-}
-
 int64_t atoi64(const char* psz)
 {
 #ifdef _MSC_VER