Remove rehash-threshold and rehash-size struct members

Remove rehash-threshold and rehash-size struct members

These parameters have no visible semantics and are hardly ever used,
so just use the default values for all hash tables.  This saves
memory, shrinks the external representation, and will improve
performance.

* src/fns.c (std_rehash_size, std_rehash_threshold): New.
(hash_index_size): Use std_rehash_threshold.  Remove table argument.
All callers updated.
(make_hash_table): Remove rehash_size and rehash_threshold args.
All callers updated.
(maybe_resize_hash_table)
(Fhash_table_rehash_size, Fhash_table_rehash_threshold):
Use std_rehash_size and std_rehash_threshold.
(Fmake_hash_table): Ignore :rehash-size and :rehash-threshold args.
* src/lisp.h (struct Lisp_Hash_Table):
Remove rehash_size and rehash_threshold fields.
(DEFAULT_REHASH_THRESHOLD, DEFAULT_REHASH_SIZE): Remove.
* src/lread.c (hash_table_from_plist): Don't read rehash-size or
rehash-threshold.
(syms_of_lread): Remove unused symbols.
* src/print.c (print_object): Don't print rehash-size or rehash-threshold.
* src/pdumper.c (dump_hash_table): Don't dump removed fields.