btrfs: zoned: no longer count fresh BG region as zone unusable

Now that we switched to write time activation, we no longer need to (and
must not) count the fresh region as zone unusable. This commit is similar
to revert of commit fa2068d7

 ("btrfs: zoned: count fresh BG
region as zone unusable").

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>