obs: drop vLLM row from dashboard (belongs in llm-d Layer 2)

The dashboard combined vLLM and EPP panels. Per the observability ownership model (llm-d/llm-d#1670), cross-component views live in llm-d Layer 2 (guides/recipes/observability) while component repos keep single-component dashboards. Retain only the EPP (llm_d_router_epp_*) panels here.

Signed-off-by: Luke Van Drie <lukevandrie@google.com>

Author: LukeAVanDrie