Remove disclaimer from Longformer docs
# What does this PR do?
This PR removes disclaimer from the Longformer docs. Checked with @patil-suraj.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?
Documentation: @sgugger
CC: @patil-suraj 
_The documentation is not available anymore as the PR was closed or merged._