core: drop useless assignments to BISON and FLEX

They were added back in 5432f26f (Adding Central config.cache options),
supposedly to be able to cache the result of configure tests, but they
were never, ever referenced anywhere in our code... Besides, we dropped
the idea of getting a configure cache long ago now (it does not work)...

They are causing spurious error messages on some distros (e.g. Fedora)
which use GNU's which (whatever package that comes from), while it is
silent on other distros (e.g. Ubuntu) which use debianutils' which.

Drop them.

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>