recovery: Drop the extern declaration of reason.

With commit ac9d94d19c5f0e8c769d2368790df679c665ea34, "reason" has been
changed from "char*" to "const char*", and it's declared in common.h.

Test: lunch aosp_dragon-userdebug; mmma bootable/recovery

Change-Id: Iedd97c21ecb11a95af1d9978e72e8b9620d46de5
diff --git a/recovery/updater/recovery_ui.cpp b/recovery/updater/recovery_ui.cpp
index 96a37db..abdb576 100644
--- a/recovery/updater/recovery_ui.cpp
+++ b/recovery/updater/recovery_ui.cpp
@@ -23,8 +23,6 @@
 #include "ui.h"
 #include "vboot_interface.h"
 
-extern char *reason;
-
 class DragonDevice : public Device {
   public:
     DragonDevice(RecoveryUI* ui) : Device(ui) { }