btrfs: make assert_rbio() to only check CONFIG_BTRFS_ASSERT

According to the description, CONFIG_BTRFS_DEBUG is only for extra
debug info, meanwhile sanity checks should be managed by
CONFIG_BTRFS_ASSERT.

There is no need to check both to enable assert_rbio().

Just remove the check for CONFIG_BTRFS_DEBUG.

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