Deprecate SchemaStatements#distinct, and make SchemaStatements#columns_for_distinct nodoc.

#distinct was previously used by #construct_limited_ids_condition, but
no more after #6792. #columns_for_distinct is just a utility method for
#construct_limited_ids_condition and shouldn't be public.
Merge pull request #10556 from Empact/deprecate-schema-statements-distinct

Deprecate SchemaStatements#distinct, and make SchemaStatements#columns_for_distinct nodoc.