Pass the verify flag to DexFileLoader utilities.

Refactoring-only change.

Also remove unused DexFilerLoader::OpenMemory.

bug: 30972906
bug: 63920015

Test: build.

Change-Id: I7cd4b5787565ab1a3457ce2be6bb14657229c550
diff --git a/runtime/class_loader_context.cc b/runtime/class_loader_context.cc
index 167533d..5510743 100644
--- a/runtime/class_loader_context.cc
+++ b/runtime/class_loader_context.cc
@@ -230,6 +230,7 @@
       // contents. So pass true to verify_checksum.
       if (!DexFileLoader::Open(location.c_str(),
                                location.c_str(),
+                               /*verify*/ true,
                                /*verify_checksum*/ true,
                                &error_msg,
                                &info.opened_dex_files)) {