test(FeeHandlerSeller.t.sol): remove unused imports

All tests pass as expected ✅

```sh
forge test -vvvv \
--match-path "test-sol/unit/common/FeeHandlerSeller.t.sol"
```

packages/protocol/test-sol/unit/common/FeeHandlerSeller.t.sol

@@ -5,14 +5,8 @@ pragma experimental ABIEncoderV2;
 // Helper contracts
 import { Test } from "celo-foundry/Test.sol";
 import { TestConstants } from "@test-sol/constants.sol";
-import { GoldTokenMock } from "@test-sol/unit/common/GoldTokenMock.sol";
-
-// Contract to test
-import "@celo-contracts/common/FeeHandler.sol";
 
-// Dependencies
-import { IRegistry } from "@celo-contracts/common/interfaces/IRegistry.sol";
-import { GoldToken } from "@celo-contracts/common/GoldToken.sol";
+import { GoldTokenMock } from "@test-sol/unit/common/GoldTokenMock.sol";
 import { FeeHandlerSeller } from "@celo-contracts/common/FeeHandlerSeller.sol";
 import { MentoFeeHandlerSeller } from "@celo-contracts/common/MentoFeeHandlerSeller.sol";
 import { UniswapFeeHandlerSeller } from "@celo-contracts/common/UniswapFeeHandlerSeller.sol";