Drop unused "reserve" fields

Probably a relic from the past, when people tried to avoid changing public
headers incompatibly to avoid causing recompilations for their
colleagues... We are long past such concerns. The talk about "extensions" in
the comment is most likely obsolete.

Change-Id: If90ca02685bf82ed529becb715b4c614287fdc57
Remove unused fields

Change-Id: Ia7d3b7e319f2a568120add15ef05915a98fe6696