fix(test): remove early exit from isFeatureEnabled fuzz test

Per maintainer feedback, removes the early exit that was skipping some
fuzz cases. The test now always runs the assertion for all fuzz inputs.

The CUSTOM_GAS_TOKEN normalization is kept to fix the original flake
in the CUSTOM_GAS_TOKEN CI environment.

Author: devin-ai-integration[bot]

packages/contracts-bedrock/test/L1/SystemConfig.t.sol

@@ -876,12 +876,6 @@ contract SystemConfig_IsFeatureEnabled_Test is SystemConfig_TestInit {
 
         vm.stopPrank();
 
-        // Skip features that are enabled in this deploy config (e.g., ETH_LOCKBOX on mainnet).
-        // Such features are not "unset" by definition.
-        if (systemConfig.isFeatureEnabled(_feature)) {
-            return;
-        }
-
         assertFalse(systemConfig.isFeatureEnabled(_feature));
     }