Remove unnecessary fallback() functions

In `FeeCollector.sol`, `PositionFactory.sol` and `AdminService.sol`, both a fallback() and receive() function are specified. No code is executed in these contracts upon receiving ether, therefore, fallback() can be omitted as it is redundant.

src/FeeCollector.sol

@@ -132,11 +132,6 @@ contract FeeCollector is Ownable {
         SafeTransferLib.safeTransfer(ERC20(_token), msg.sender, withdrawAmt);
     }
 
-    /**
-     * @notice Executes when native is sent to this contract through a non-existent function.
-     */
-    fallback() external payable { } // solhint-disable-line no-empty-blocks
-
     /**
      * @notice Executes when native is sent to this contract with a plain transaction.
      */

src/PositionFactory.sol

@@ -81,11 +81,6 @@ contract PositionFactory is Ownable {
         SafeTransferLib.safeTransfer(ERC20(_token), msg.sender, balance);
     }
 
-    /**
-     * @notice Executes when native is sent to this contract through a non-existent function.
-     */
-    fallback() external payable { } // solhint-disable-line no-empty-blocks
-
     /**
      * @notice Executes when native is sent to this contract with a plain transaction.
      */

src/services/AdminService.sol

@@ -36,11 +36,6 @@ contract AdminService {
         SafeTransferLib.safeTransfer(ERC20(_token), msg.sender, balance);
     }
 
-    /**
-     * @notice Executes when native is sent to this contract through a non-existent function.
-     */
-    fallback() external payable { } // solhint-disable-line no-empty-blocks
-
     /**
      * @notice Executes when native is sent to this contract with a plain transaction.
      */