config: ext4: drop option to set maximum number of inodes

There is very little practical use to limit the number of available inodes on
an ext4 filesystem and the make_ext4fs utility is able to calculate useful
defaults by itself.

Drop the option to make resulting ext4 filesystems more flexible by default.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Acked-by: Michael Heimpold <mhei@heimpold.de>