chore: prune unused eslint suppressions for CurrencyRateController

Removed unused suppressions after refactoring:
- no-negated-condition: no longer needed
- no-restricted-syntax: no longer needed
- @typescript-eslint/explicit-function-return-type: reduced count from 3 to 1

Author: Prithpal-Sooriya

eslint-suppressions.json

@@ -187,13 +187,7 @@
   },
   "packages/assets-controllers/src/CurrencyRateController.ts": {
     "@typescript-eslint/explicit-function-return-type": {
-      "count": 3
-    },
-    "no-negated-condition": {
       "count": 1
-    },
-    "no-restricted-syntax": {
-      "count": 3
     }
   },
   "packages/assets-controllers/src/DeFiPositionsController/DeFiPositionsController.test.ts": {
@@ -2101,4 +2095,4 @@
       "count": 1
     }
   }
-}
+}