Remove legacy 'without packer' test scenarios and using_packer? references

Since React on Rails 16 is Shakapacker-only, the following test scenarios
are no longer relevant and have been removed:

- 'without packer enabled' shared context and tests
- Test context for "when not using packer" in react_client_manifest_file_path
- References to using_packer? method which was removed

This eliminates test failures for scenarios that can't happen in React on Rails 16.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/react_on_rails/utils_spec.rb

@@ -35,18 +35,6 @@ module ReactOnRails
       end
     end
 
-    shared_context "without packer enabled" do
-      before do
-        allow(ReactOnRails).to receive_message_chain(:configuration, :generated_assets_dir)
-          .and_return("public/webpack/dev")
-        allow(described_class).to receive(:gem_available?).with("shakapacker").and_return(false)
-      end
-
-      it "does not use packer" do
-        expect(ReactOnRails::PackerUtils.packer).to be_nil
-      end
-    end
-
     def mock_bundle_in_manifest(bundle_name, hashed_bundle)
       mock_manifest = instance_double(Shakapacker::Manifest)
       allow(mock_manifest).to receive(:lookup!)
@@ -132,12 +120,6 @@ def mock_dev_server_running
             end
           end
         end
-
-        context "without a packer enabled" do
-          include_context "without packer enabled"
-
-          it { is_expected.to eq(File.expand_path(File.join(Rails.root, "public/webpack/dev/webpack-bundle.js"))) }
-        end
       end
 
       describe ".source_path_is_not_defined_and_custom_node_modules?" do
@@ -500,7 +482,6 @@ def mock_dev_server_running
         let(:public_output_path) { "/path/to/public/webpack/dev" }
 
         before do
-          allow(ReactOnRails::PackerUtils).to receive(:using_packer?).and_return(true)
           allow(ReactOnRails::PackerUtils.packer).to receive_message_chain("config.public_output_path")
             .and_return(Pathname.new(public_output_path))
           allow(ReactOnRails::PackerUtils.packer).to receive_message_chain("config.public_path")
@@ -537,19 +518,6 @@ def mock_dev_server_running
           end
         end
       end
-
-      context "when not using packer" do
-        before do
-          allow(ReactOnRails::PackerUtils).to receive(:using_packer?).and_return(false)
-          allow(described_class).to receive(:generated_assets_full_path)
-            .and_return("/path/to/generated/assets")
-        end
-
-        it "returns joined path with generated_assets_full_path" do
-          expect(described_class.react_client_manifest_file_path)
-            .to eq("/path/to/generated/assets/react-client-manifest.json")
-        end
-      end
     end
 
     describe ".react_server_client_manifest_file_path" do