package/lvm2: drop optional readline dependency

lvm2 is licensed under GPL-2.0, which is not license compatible with
readline (GPL-3.0+), so drop the optional dependency and add a comment
explaining why.

Notice: The readline support is only used when the raw lvm tool is called
without arguments.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>