Remove KeyedAccount in builtin program "BPF loader" (#24381)

* Uses transaction_context.get_key_of_account_at_index() when only the key is needed.

* Uses "!instruction_context.is_signer()" instead of ".signer_key().is_none()".

* Uses instruction_context.get_index_in_transaction() to detect account aliasing.

* Makes sure that there is only one KeyedAccount at a time.
Everywhere except for DeployWithMaxDataLen and Upgrade.

* Makes sure that there is only one KeyedAccount at a time.
In DeployWithMaxDataLen and Upgrade.

* Replaces KeyedAccount by BorrowedAccount.

* Removes unused code.