Documentation: filesystems: remove reminiscences of POHMELFS

The POHMELFS filesystem was removed in 2012 by commit 67435319
("staging: pohmelfs: remove drivers/staging/pohmelfs") promising that
a newer version will be included to the kernel, but unfortunately
it didn't happen.

Since likely any delopment of the filesystem is halted, the change removes
the abandoned POHMELFS documentation from the kernel tree.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>