filesystem: De-globalize registered_cache_union

We can just return a new instance of this when it's requested. This only
ever holds pointers to the existing registed caches, so it's not a large
object. Plus, this also gets rid of the need to keep around a separate
member function just to properly clear out the union.

Gets rid of one of five globals in the filesystem code.

src/core/crypto/key_manager.cpp

@@ -794,7 +794,7 @@ void KeyManager::DeriveBase() {
 
 void KeyManager::DeriveETicket(PartitionDataManager& data) {
     // ETicket keys
-    const auto es = Service::FileSystem::GetUnionContents()->GetEntry(
+    const auto es = Service::FileSystem::GetUnionContents().GetEntry(
         0x0100000000000033, FileSys::ContentRecordType::Program);
 
     if (es == nullptr)