update_engine: cxx20 support.

Get rid of user declared ctor from InstallPlan.
In C++20, one of the requirements to have a designated initializer
is that the struct does not have user declared ctor.

BUG=b:217226507
TEST=tryjob

Change-Id: I2d6cb1652c4c40f4316d79a7ad6e766fd408ee6b
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/4343353
Tested-by: Hidehiko Abe <hidehiko@chromium.org>
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Qijiang Yƫki Ishii <fqj@google.com>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/payload_consumer/install_plan.h b/payload_consumer/install_plan.h
index 256fa7b..ac24121 100644
--- a/payload_consumer/install_plan.h
+++ b/payload_consumer/install_plan.h
@@ -46,8 +46,6 @@
 };
 
 struct InstallPlan {
-  InstallPlan() = default;
-
   bool operator==(const InstallPlan& that) const;
   bool operator!=(const InstallPlan& that) const;