- Changing the data type of the global variable any_db from
  LEX_CSTRING to Lex_ident_db

- Removing the dependency on system_charset_info from
  Lex_ident_fs::check_body(), using my_charset_utf8mb3_general_ci directly,
  because system_charset_info is initialized much later than any_db.
  system_charset_info cannot be changed dynamically any way.

- Removing the unsed old code from Lex_ident_fs::check_body().
  This code was last used in MySQL-4.0 and won't be used in the future.