btrfs: zoned: drop no longer valid write pointer check

There is a check of the write pointer vs the zone size to reject an invalid
write pointer. However, as of now, we have RAID0/RAID10 on the zoned
mode, we can have a block group whose size is larger than the zone size.

As an equivalent check against the block group's zone_capacity is already
there, we can just drop this invalid check.

Fixes: 568220fa ("btrfs: zoned: support RAID0/1/10 on top of raid stripe tree")
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>