exchange: remove check for 'format' key
When the 'kwargs' variable was added in
12f161f08d74 (bundle2: allow
pulling changegroups using bundle2, 2014-04-01), it could contain only
'bundlecaps', 'common' and 'heads', so the check for 'format' would
always be false. Since then, _pullbundle2extraprepare() has been added
for hooks, but it seems unlikely that they would a 'format' key.
templates-default: do not show description or summary if empty
changeset_printer shows description only if ctx.description().strip() is not
empty. The default template should do the same way.