Remove `%thong' support as it is undocumented, unused, duplicates

`%token's job, and creates useless e-mail traffic with people who
want to know what it is, why it is undocumented, unused, and
duplicates `%token's job.
* src/reader.c (parse_thong_decl): Remove.
* src/options.c (option_table): Remove "thong".
* src/lex.h (tok_thong): Remove.