net: nfs: Drop CONFIG_NFS_READ_SIZE

In the general case, CONFIG_NFS_READ_SIZE is unchanged from the default
of 1024.  There are in fact no in-tree users that increase this size.
Adjust the comment to reflect what could be done in the future in
conjunction with CONFIG_IP_DEFRAG.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>