The predicate dict_table_is_discarded() checks whether
ALTER TABLE…DISCARD TABLESPACE has been executed.

Replace most occurrences of dict_table_is_discarded() with
checks of dict_table_t::space. A few checks for the flag
DICT_TF2_DISCARDED are necessary; write them inline.

Because !is_readable() implies !space, some checks for
dict_table_is_discarded() were redundant.