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>

cmake/kconfig.cmake

@@ -23,7 +23,6 @@ endif()
 set(ENV{srctree}            ${ZEPHYR_BASE})
 set(ENV{KERNELVERSION}      ${PROJECT_VERSION})
 set(ENV{KCONFIG_CONFIG}     ${DOTCONFIG})
-set(ENV{KCONFIG_AUTOHEADER} ${AUTOCONF_H})
 
 # Set environment variables so that Kconfig can prune Kconfig source
 # files for other architectures