bcachefs: bch2_bucket_alloc_new_fs() no longer depends on bucket marks

bcachefs: bch2_bucket_alloc_new_fs() no longer depends on bucket marks

Now that bch2_bucket_alloc_new_fs() isn't looking at bucket marks to
decide what buckets are eligible to allocate, we can clean up the
filesystem initialization and device add paths. Previously, we had to
use ancient code to mark superblock/journal buckets in the in memory
bucket marks as we allocated them, and then zero that out and re-do that
marking using the newer transational bucket mark paths. Now, we can
simply delete the in-memory bucket marking.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>