update_engine: remove system_id and system_version

These two variables and their use cases was introduced for android, but
eventually android abandon it for some other solution. So this are stale
now.

Also removed the implemnation of ImageProperties for Android as it
doesn't seem Android is actually using it.

BUG=b:171829801
TEST=unittests

Change-Id: Ic793cfa5031d69b5390acefb1b8cd75291708890
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2505885
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Tianjie Xu <xunchang@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
diff --git a/omaha_request_params.cc b/omaha_request_params.cc
index 5a48720..ce6fd27 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -78,14 +78,7 @@
   LOG(INFO) << "Running from channel " << image_props_.current_channel;
 
   os_platform_ = constants::kOmahaPlatformName;
-  if (!image_props_.system_version.empty()) {
-    if (app_version == "ForcedUpdate") {
-      image_props_.system_version = app_version;
-    }
-    os_version_ = image_props_.system_version;
-  } else {
-    os_version_ = OmahaRequestParams::kOsVersion;
-  }
+  os_version_ = OmahaRequestParams::kOsVersion;
   if (!app_version.empty())
     image_props_.version = app_version;