MDEV-9465 The constructor StringBuffer(const char *str, size_t length…

…, const CHARSET_INFO *cs) looks suspicious

Removing the suspicious constructor, it's not used in the code anyway.

sql/sql_string.h

@@ -649,11 +649,6 @@ class StringBuffer : public String
   {
     length(0);
   }
-  StringBuffer(const char *str, size_t length_arg, CHARSET_INFO *cs)
-    : String(buff, buff_sz, cs)
-  {
-    set(str, length_arg, cs);
-  }
 };