Bug 47426: Omit id member so that data-mw attribute is deterministic

The non-deterministic id member in data-mw made the entire attribute
serialization non-deterministic. The attribute difference triggers a DOM diff
and full serialization rather than selective serialization. Full serialization
introduces avoidable whitespace diffs.

No differences in parserTests and tested with roundtrip-test on Obama.

Change-Id: Ic615b11b5bc7cd7cb83a59ffa9fcccb135909a28