Fix duplicate dictionary key 31 in XID_MESSAGES

Remove duplicate key 31 from XID_MESSAGES dictionary in gpu_xid_injector.py.
Key 31 was defined twice:
- Line 92: "MMU Error" (Memory Subsystem section)
- Line 113: "GPU stopped responding" (Graphics section)

Removed the duplicate from Graphics section as the MMU Error definition
is the primary meaning and the comment indicated it can be both contexts.

Fixes ruff pre-commit error: F601 Dictionary key literal `31` repeated
Signed-off-by: Oviya Seeniraj <[email protected]>

tests/fault_tolerance/hardware/fault-injection-service/agents/gpu-fault-injector/gpu_xid_injector.py

@@ -110,7 +110,6 @@
     57: "Clocks Event: Power limit exceeded",
     # Common Graphics XIDs (often seen in test environments)
     13: "Graphics Engine Exception",
-    31: "GPU stopped responding",  # Can be both MMU or timeout context
     45: "Preemptive Cleanup, due to previous errors",
     69: "Graphics Exception: Class Error",
 }