Remove ProxyResolver from update_engine

Current ProxyResolver code only supports no-proxy. Therefore it is
barely any use. Remove current implementation for a better one.

Test: th
Bug: 235816007
Change-Id: Ieb46dedf6c6ea86b11c81d7691c2adb578d3d97d
diff --git a/common/constants.h b/common/constants.h
index 8c07fcf..53c5bba 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -191,6 +191,9 @@
 static constexpr const auto& kXGoogleUpdateUpdater = "X-Goog-Update-Updater";
 static constexpr const auto& kXGoogleUpdateSessionId = "X-Goog-SessionId";
 
+// Proxy URL for direction connection
+static constexpr const auto& kNoProxy = "direct://";
+
 // A download source is any combination of protocol and server (that's of
 // interest to us when looking at UMA metrics) using which we may download
 // the payload.