Move string_printf to maxbase

Can be used in tests.

include/maxscale/utils.hh

@@ -137,15 +137,6 @@ inline std::vector<std::string> strtok(std::string str, const char* delim)
     return rval;
 }
 
-/**
- * Format parameters to a string. Uses printf-formatting.
- *
- * @param format Format string
- * @param ... Items to convert according to format string
- * @return The result string
- */
-std::string string_printf(const char* format, ...) mxb_attribute((format (printf, 1, 2)));
-
 /**
  * @class CloserTraits utils.hh <maxscale/utils.hh>
  *