components: default the user-defined empty ctor and dtor where possible

I need an owner's approval please for changes under:
components/policy

This is a mechanical change. There are a few instances (but not all)
of empty user-defined constructors and destructors which can be
defaulted in the /components code.

A few of these have been missed by previous CLs and are being changed
now.

This CL was uploaded by git cl split.

R=hendrich@chromium.org

Bug: 374110115
Change-Id: Iea937df9dcf99613aa16032765d216d7a11da609
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5963134
Commit-Queue: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Auto-Submit: Sorin Jianu <sorin@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1374546}


CrOS-Libchrome-Original-Commit: 3fa122a7be5665398119f96dab56e55607dec9f5

Author: sorinj

components/policy/core/common/schema_registry.cc

@@ -16,9 +16,9 @@
 
 namespace policy {
 
-SchemaRegistry::Observer::~Observer() {}
+SchemaRegistry::Observer::~Observer() = default;
 
-SchemaRegistry::InternalObserver::~InternalObserver() {}
+SchemaRegistry::InternalObserver::~InternalObserver() = default;
 
 SchemaRegistry::SchemaRegistry() : schema_map_(new SchemaMap) {
   for (int i = 0; i < POLICY_DOMAIN_SIZE; ++i)