[autoroll] Make meta-config generator remove old configs first
[autoroll] Make meta-config generator remove old configs first

Otherwise, we'll leave them behind.

Change-Id: Ie3e5c19f28a5c6af31fbe0ee14d7b38c79529362
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/543477
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot