Remove gen_vault_signer_key for test

[fanatid]

Is there any reason for default Pubkey in gen_vault_signer_key?

My use case:
I'm trying to process instructions in Rust code with real data from the network (1. accounts data received from validators, so they are real 2. fuzz is used because only this feature process spl_token instructions instead of cross program invocations). Because gen_vault_signer_key produce default Pubkey I have a problem with SettleFunds instruction, I need to pass vault signer as default Pubkey for successful validation from serum dex and at the same time need to pass correct vault signer with real Pubkey for successful validation from spl_token. Currently, I changed owner pubkey for vault addresses but looks like a real in the key returned by gen_vault_signer_key.