[ROCm] Validate block_size for explicitly selected attention backends (#36846)

Signed-off-by: Andreas Karatzas <akaratza@amd.com>

vllm/platforms/rocm.py

@@ -438,8 +438,6 @@ class RocmPlatform(Platform):
         device_capability = cls.get_device_capability()
         assert device_capability is not None
 
-        attn_selector_config = attn_selector_config._replace(block_size=None)
-
         # First try checking just the selected backend, if there is one.
         if selected_backend is not None:
             try: