btrfs: drop unused argument of calcu_metadata_size()

calcu_metadata_size() has a "reserve" argument, but the only caller always
set it to "1". The other usage (reserve = 0) is dropped by a commit
0647bf56 ("Btrfs: improve forever loop when doing balance relocation"),
which is more than 10 years ago. Drop the argument and simplify the code.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>