MDEV-31992 Automatic conversion from LEX_STRING to LEX_CSTRING

- Adding automatic conversion operator from LEX_STRING to LEX_CSTRING
  Now a LEX_STRING can be passed directly to any function expecting
  a LEX_CSTRING parameter passed by value or by reference.
- Removing a number of duplicate methods accepting LEX_STRING.
  Now the code used the LEX_CSTRING version.

sql/sql_string.h

@@ -1052,13 +1052,6 @@ public:
   }
 
   // Append with optional character set conversion from ASCII (e.g. to UCS2)
-  bool append(const LEX_STRING *ls)
-  {
-    DBUG_ASSERT(ls->length < UINT_MAX32 &&
-                ((ls->length == 0 && !ls->str) ||
-                 ls->length == strlen(ls->str)));
-    return append(ls->str, (uint32) ls->length);
-  }
   bool append(const LEX_CSTRING *ls)
   {
     DBUG_ASSERT(ls->length < UINT_MAX32 &&