lftp: remove unneeded config options

When installation prefix is specified in "--with-readline" config
option, lftp's build systems figures out include dir and linker flags.
So don't specify --with-readline-{inc,libs} options which are anyways
get ignored by build system.

Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>