refactor: remove nofallback parameter

Remove the nofallback input parameter as it's no longer needed. The action
now assumes that a builder is already configured and available.

Changes:
- Remove nofallback from action.yml inputs
- Remove nofallback from Inputs interface and getInputs function
- Update tests to remove nofallback references
- Also remove setup-only and buildx-version from action.yml (already removed from code)

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

Co-Authored-By: Claude <noreply@anthropic.com>

src/__tests__/blacksmith-builder.test.ts

@@ -34,7 +34,6 @@ describe('reportBuildMetrics', () => {
   beforeEach(() => {
     jest.clearAllMocks();
     mockInputs = {
-      nofallback: false,
       setupOnly: false,
       platforms: []
     };