arc: remove dummy target

TARGET_DUMMY was introduced to resolve the same problem as commit 
"arch: Make board selection choices optional"
http://git.denx.de/?p=u-boot.git;a=commit;h=a26cd04920dc069fd6e91abb785426cf6c29f45f

Latter implementation is much cleaner and appropriate.

And anyways TARGET_DUMMY is no longer required.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

arch/arc/Kconfig

@@ -134,14 +134,6 @@ choice
 	prompt "Target select"
 	optional
 
-config TARGET_DUMMY
-	bool "Dummy target"
-	help
-	  Please select one of real target boards below!
-	  This target is only meant to force "makedefconfig" to put
-	  TARGET_xxx in defconfig even this is the first target from the list
-	  below.
-
 config TARGET_TB100
 	bool "Support tb100"