zephyr: hello-world: remove BUILD_TIMESTAMP from prj.conf

This feature has been deleted in Zephyr upstream. The project won't
build with this line in anymore due to Kconfig errors.

Signed-off-by: Marti Bolivar <marti@foundries.io>
diff --git a/samples/zephyr/hello-world/prj.conf b/samples/zephyr/hello-world/prj.conf
index 18c4651..cc67406 100644
--- a/samples/zephyr/hello-world/prj.conf
+++ b/samples/zephyr/hello-world/prj.conf
@@ -1,6 +1,5 @@
 # Print a banner on the UART on startup.
 CONFIG_BOOT_BANNER=y
-CONFIG_BUILD_TIMESTAMP=y
 
 # Enable console and printk()
 CONFIG_PRINTK=y