Cleanup Lex_ident_fs::check_body()

The main goal of this task is to change the data type of the global variable any_db from LEX_CSTRING to Lex_ident_db, to make upcoming changes easier:

-const LEX_CSTRING any_db= {STRING_WITH_LEN("*any*")};

+const Lex_ident_db any_db(STRING_WITH_LEN("*any*"));

But most changes have to be done in Lex_ident_fs::check_body(), hence the task title.

Lex_ident_fs::check_body() depends from system_charset_info, which is initialized after any_db during the server startup.
So we need to change system_charset_info to direct use of my_charset_utf8mb3_general_ci.

Additionally, let's remove the unused code from Lex_ident_fs::check_body().
We always compile in the way that this condition evaluates to true:

#if defined(USE_MB) && defined(USE_MB_IDENT)

...

#else

...

#endif

There is no sence to keep the old MySQL-4.0 code from the "#else" branch.