drop APPLY_CACHE and refactor APPLY_EXT

Drop support for sideloading OTA packages of the cache partition (a
half-solution that's long since been deprecated by "adb sideload").
Refactor the code to sideload OTA packages from SD cards: remove the
installation code from the file browser.

Change-Id: Id0dff6b27c4a5837546f174f50e2e1d0379c43db
diff --git a/device.h b/device.h
index df71377..57ec3fc 100644
--- a/device.h
+++ b/device.h
@@ -65,7 +65,8 @@
     //   - invoke a specific action (a menu position: any non-negative number)
     virtual int HandleMenuKey(int key, int visible) = 0;
 
-    enum BuiltinAction { NO_ACTION, REBOOT, APPLY_EXT, APPLY_CACHE,
+    enum BuiltinAction { NO_ACTION, REBOOT, APPLY_EXT,
+                         APPLY_CACHE,   // APPLY_CACHE is deprecated; has no effect
                          APPLY_ADB_SIDELOAD, WIPE_DATA, WIPE_CACHE,
                          REBOOT_BOOTLOADER, SHUTDOWN };