fix: drop duplicate claude-sonnet-4-5 from model_cost.json

Upstream already has claude-sonnet-4-5 at line 54; my surgical append
re-added it. check-json hook caught the duplicate. Removing the
re-added block leaves upstream's entry intact.

Author: steventohme

model_cost/model_cost.json

@@ -191,10 +191,6 @@
     "input_token_price_per_million": 15.00,
     "output_token_price_per_million": 75.00
   },
-  "claude-sonnet-4-5": {
-    "input_token_price_per_million": 3.00,
-    "output_token_price_per_million": 15.00
-  },
   "claude-haiku-4-5": {
     "input_token_price_per_million": 0.80,
     "output_token_price_per_million": 4.00