btrfs-progs: convert: remove redundant check

The check for sectorsize is properly done in do_convert, remove the
BUG_ON.

Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-convert.c

@@ -1359,7 +1359,6 @@ static int migrate_super_block(int fd, u64 old_bytenr, u32 sectorsize)
 	u32 len;
 	u32 bytenr;
 
-	BUG_ON(sectorsize < sizeof(*super));
 	buf = malloc(sizeof(*buf) + sectorsize);
 	if (!buf)
 		return -ENOMEM;