bcachefs: Delete backpointers check in try_alloc_bucket()

bcachefs: Delete backpointers check in try_alloc_bucket()

try_alloc_bucket() has a "safety" check, which avoids allocating a
bucket if there's any backpointers present.

But backpointers are not the source of truth for live data in a bucket,
the bucket sector counts are; this check was fairly useless, and we're
also deferring backpointers checks from fsck to runtime in the near
future.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>