kconfig: Don't set the deprecated env var KCONFIG_AUTOHEADER

KCONFIG_AUTOHEADER is an environment variable that was used by the C
Kconfig tools to determine where the autconf.h file is. After the
migration to python this environment variable became unused.

This patch removes the deprecated env var.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>