docs: sysctl/fs: remove references to inode-max

inode-max was removed in 2.3.20pre1, remove references to it in the
sysctl documentation.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20220930102937.135841-2-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>