ext4: remove ext4_block_group and ext4_block_group_offset declaration

For ext4_block_group and ext4_block_group_offset, there are only
declaration without definition. Just remove them.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230603150327.3596033-7-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>