packages/flex: remove unnecessary BR2_PACKAGE_FLEX_LIBFL

The BR2_PACKAGE_FLEX_LIBFL knob was removed by
bd72a13a so remove it from the packages
that used it since it's completely unnecessary.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>