Fix assumptions about ImportRefUnused in lexical lookups (#3784)

I'd added these trying to catch cases where ImportRefUnused might be
returned, but I believe these are incorrect. Mistakes should still be
caught because an ImportRefUnused will lead to errors in other handling.
Removing the TryResolveImportRefUnused should clarify the semantics of
when an ImportRefUnused is expected.

Author: jonmeow

toolchain/check/context.cpp

@@ -216,11 +216,7 @@ auto Context::LookupNameInDecl(Parse::NodeId /*node_id*/, SemIR::NameId name_id,
     //    In this case, we're not in the correct scope to define a member of
     //    class A, so we should reject, and we achieve this by not finding the
     //    name A from the outer scope.
-    auto result = scope_stack().LookupInCurrentScope(name_id);
-    if (result.is_valid()) {
-      TryResolveImportRefUnused(*this, result);
-    }
-    return result;
+    return scope_stack().LookupInCurrentScope(name_id);
   } else {
     // We do not look into `extend`ed scopes here. A qualified name in a
     // declaration must specify the exact scope in which the name was originally
@@ -255,7 +251,6 @@ auto Context::LookupUnqualifiedName(Parse::NodeId node_id,
   }
 
   if (lexical_result.is_valid()) {
-    TryResolveImportRefUnused(*this, lexical_result);
     return lexical_result;
   }