btrfs: kill min_allocable_bytes in inc_block_group_ro

This is a relic from a time before we had a proper reservation mechanism
and you could end up with really full chunks at chunk allocation time.
This doesn't make sense anymore, so just kill it.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>