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/context.ts

@@ -57,7 +57,6 @@ export interface Inputs {
   target: string;
   ulimit: string[];
   'github-token': string;
-  nofallback: boolean;
 }
 
 export async function getInputs(): Promise<Inputs> {
@@ -94,8 +93,7 @@ export async function getInputs(): Promise<Inputs> {
     tags: Util.getInputList('tags'),
     target: core.getInput('target'),
     ulimit: Util.getInputList('ulimit', {ignoreComma: true}),
-    'github-token': core.getInput('github-token'),
-    nofallback: core.getBooleanInput('nofallback')
+    'github-token': core.getInput('github-token')
   };
 }