Remove uses of #[allow(dead_code) in favor of names starting with _

[alamb]

TLDR is that we have some code (such as referenced below) that uses #[allow(dead_code)] to permit a field that is only present so its Drop (destructor) can run.

          Apparently this can be also written without disabling a lint:

    _reservation: MemoryReservation,

• #[allow(dead_code)] has disadvantage that it can be retained in the code even if the field becomes used
• _reservation has disadvantage that it doesn't look pretty

Originally posted by @findepi in #13227 (comment)

@drauschenbach notes: #13227 (comment)

Pretty or not, _reservation is the language-appropriate idiom.

Thus it would be great to update the code to use _reservation in the case above, and potentially look in the codebase for other similar uses of #[allow(dead_code)] and use the _ prefix

[ding-young]

take