Cleanup: Remove handler::update_table_comment()

The only call of the virtual member function
handler::update_table_comment() was removed in
commit 82d28fada7 (MySQL 5.5.53)
but the implementation was not removed.

The only non-trivial implementation was for InnoDB. The information
is now returned via handler::get_foreign_key_create_info() and
ha_statistics::delete_length.

sql/handler.h

@@ -2,7 +2,7 @@
 #define HANDLER_INCLUDED
 /*
    Copyright (c) 2000, 2019, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2019, MariaDB
+   Copyright (c) 2009, 2021, MariaDB
 
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
@@ -3363,8 +3363,6 @@ public:
   /* end of the list of admin commands */
 
   virtual int indexes_are_disabled(void) {return 0;}
-  virtual char *update_table_comment(const char * comment)
-  { return (char*) comment;}
   virtual void append_create_info(String *packet) {}
   /**
     If index == MAX_KEY then a check for table is made and if index <