Revert "PG::init: clear rollback info for backfill as well"

Log here is already empty, need to rollback the log when we start
removal in on_removal().

This reverts commit 1e69ff2f3c7940d3969f5357b5aa1463015213e2.
ReplicatedPG::on_removal: clear rollback info

Fixes: #9293
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2561 from athanatos/wip-9293

Wip 9293

Reviewed-by: Sage Weil <sage@redhat.com>