Remove arc directory support

In the server we have obsolete code to handle 'arc' directories in the following places
sql/sql_db.cc: find_db_tables_and_rm_known_files()
sql/sql_db.cc: mysql_rm_arc_files()
sql/parse_file.cc:rename_in_schema_file()

Remove this code and instead fix so that mysql_upgrade removes any old arc directories,
preferably with the code used by mysql_rm_arc_files().

There is no logic in the current code that will delete the 'arc' files when renaming a trigger file!