Remove ImportRefUsed (#3934)

This was to track use of a declaration after import, prior to a
redeclaration. Per [discussion on
Discord](https://discord.com/channels/655572317891461132/1217182321933815820/1236016521059237962),
we likely don't need this check due to the change in behavior of
`extern`.

Rather than potentially getting one of many `extern` decls and depending
on it by accident, it is now planned to be _required_ to be imported,
and the library doing a non-`extern` decl must _know_ it's importing the
`extern` decl. The stricter requirement on the library means it now
seems more reasonable to use the `extern` decl.

So kind of rolling back #3831, though keeping `ImportIRInstId` (at least
for now) and keeping `Loaded`/`Unloaded` terminology (seems a nicer
fit).

toolchain/check/decl_name_stack.cpp

@@ -195,8 +195,7 @@ auto DeclNameStack::ApplyNameQualifierTo(NameContext& name_context,
   if (TryResolveQualifier(name_context, loc_id)) {
     // For identifier nodes, we need to perform a lookup on the identifier.
     auto resolved_inst_id = context_->LookupNameInDecl(
-        name_context.loc_id, name_id, name_context.target_scope_id,
-        /*mark_imports_used=*/false);
+        name_context.loc_id, name_id, name_context.target_scope_id);
     if (!resolved_inst_id.is_valid()) {
       // Invalid indicates an unresolved name. Store it and return.
       name_context.state = NameContext::State::Unresolved;