[AMD-NPU] Updating VitisAI readme to point to olive-recipes

[poganesh]

Describe your changes

This PR removes the AMD NPU VitisAI recipe configs from Olive/examples/vitisai and updates relevant README files to point to the olive-recipes repository, where these recipes are now maintained.

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.
• Is this PR including examples changes? If yes, please remember to update example documentation in a follow-up PR.

(Optional) Issue link

[VishalX]

@poganesh / @jambayk, the requirements file is also removed from by this PR. Shouldn't we have this here? If not, we have to make sure the instructions in README is clear.