incremental OTAs should remove /system/recovery.img

This file has been replaced by the recovery_from_boot.p patch file,
but needs to be removed from existing devices if it exists.

tools/releasetools/ota_from_target_files

@@ -618,7 +618,8 @@ def WriteIncrementalOTAPackage(target_zip, source_zip, output_zip):
   script.Print("Removing unneeded files...")
   script.DeleteFiles(["/"+i[0] for i in verbatim_targets] +
                      ["/"+i for i in sorted(source_data)
-                            if i not in target_data])
+                            if i not in target_data] +
+                     ["/system/recovery.img"])
 
   if updating_boot:
     # Produce the boot image by applying a patch to the current