fix: L04 - Remove repeated function

[chrismaree]

OZ identified:

The _toBytes32 [internal function](https://github.com/across-protocol/contracts/blob/401e24ccca1b3af919dd521e58acd445297b65b6/contracts/erc7683/ERC7683OrderDepositor.sol#L355) of the ERC7683OrderDepositor contract has the same logic as the imported toBytes32 [function](https://github.com/across-protocol/contracts/blob/401e24ccca1b3af919dd521e58acd445297b65b6/contracts/libraries/AddressConverters.sol#L23) of the AddressConverters library. Both the _toBytes32 and toBytes32 functions are [used](https://github.com/across-protocol/contracts/blob/401e24ccca1b3af919dd521e58acd445297b65b6/contracts/erc7683/ERC7683OrderDepositor.sol#L229-L247) throughout the ERC7683OrderDepositor contract, which lessens code clarity.

In order to improve the overall code quality and avoid having duplicated code, consider removing the _toBytes32 function and using the library function instead.

This PR addresses this as well as the same logic/problem for addresses in the same file.