[Docs] Remove PDF build from Readtehdocs (vllm-project#9347)

.readthedocs.yaml

@@ -13,10 +13,10 @@ sphinx:
    fail_on_warning: true
 
 # If using Sphinx, optionally build your docs in additional formats such as PDF
-formats:
-   - pdf
+formats: []
 
 # Optionally declare the Python requirements required to build your docs
 python:
    install:
    - requirements: docs/requirements-docs.txt
+