format: remove sidedata format variant
This format variant can only exist when also using revlog v2, so it's useless
now.
Differential Revision: https://phab.mercurial-scm.org/D10113
revlogv2: allow upgrading to v2
Revlogv2 implies sidedata.
Right now sidedata is not really used in production, and Revlogv2 will be used
for the first production-ready version of sidedata support.
Differential Revision: https://phab.mercurial-scm.org/D9844