km/common: drop unneeded std* environment variables

These variables are only used if CONFIG_SYS_CONSOLE_IS_IN_ENV
is set. This isn't the case, so we can drop them safely.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>