binutils: drop target flex/bison from build dependencies

binutils: drop target flex/bison from build dependencies

This lengthens dependency chains unnecessarily;
also -native versions are already included in DEPENDS via binutils.inc:

DEPENDS = "flex-native bison-native zlib-native gnu-config-native autoconf-native"

I have also checked that nothing in binutils needs anything from target
sysroots of flex/bison:
https://lists.openembedded.org/g/openembedded-core/message/155131

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>