UnionAsMapOptions.omit_nulls and .omit_defaults are never read

[lalinsky]

UnionAsMapOptions declares two fields that are never read, in either direction:

pub const UnionAsMapOptions = struct {
    key: union(enum) { field_name, field_name_prefix: u8, field_index },
    omit_nulls: bool = true,
    omit_defaults: bool = false,
};

grep for omit_nulls/omit_defaults in src/union.zig finds only these two declarations. They are copy-paste from StructAsMapOptions, where both are genuinely used by isStructFieldUsed on the encode side.

They are also meaningless here: packUnionAsMap always writes packMapHeader(writer, 1) — exactly one entry, the active variant — so there is nothing to omit.

Same category as the dead surface removed in #27. Two lines, and no behaviour change since nothing reads them.

Noted while adding skip_unknown_fields in #28 and left out to keep that PR focused.