microblaze: Do not force saving variables to flash

There is no reason to save variables to flash only.
Select option via Kconfig instead.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index dcc502b..c564593 100644
--- a/arch/microblaze/Kconfig
+++ b/arch/microblaze/Kconfig
@@ -15,7 +15,6 @@
 	select OF_CONTROL
 	select DM
 	select DM_SERIAL
-	select ENV_IS_IN_FLASH
 	select SYSRESET
 
 endchoice