zephyr: remove ZEPHYR_BASE "bounce option" from Kconfig

These are not necessary anymore; kconfiglib evaluates $FOO in the
environment.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index d00cbbc..feead57 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -271,8 +271,4 @@
 
 comment "Zephyr configuration options"
 
-config ZEPHYR_BASE
-	string
-	option env="ZEPHYR_BASE"
-
 source "$ZEPHYR_BASE/Kconfig.zephyr"