Variable names that start with an underscore should be removed

[Baelyk]

Variables that start with underscores (e.g. _isbn in isbn-verify) should be removed. The reason they are there is to silence unused variable warnings, and there is a better way of doing that (see #442).

Exercises I know of from working on #474:

• diffie-hellman
• isbn-verifier
• series

[petertseng]

I think that is in fact all of them.

$ git grep 'fn.*(_[^:]'
exercises/diffie-hellman/src/lib.rs:pub fn private_key(_p: u64) -> u64 {
exercises/diffie-hellman/src/lib.rs:pub fn public_key(_p: u64, _g: u64, _a: u64) -> u64 {
exercises/diffie-hellman/src/lib.rs:pub fn secret(_p: u64, _b_pub: u64, _a: u64) -> u64 {
exercises/isbn-verifier/src/lib.rs:pub fn is_valid_isbn(_isbn: &str) -> bool {
exercises/series/src/lib.rs:pub fn series(_digits: &str, _len: usize) -> Vec<String> {

[petertseng]

NOTE: It is okay that simple-linked-list has an _item. See commit message

     2. Because the element in push is generic over T, which is not bounded
        by Debug, we can't emit it in an unimplemented! statement.
        We therefore fall back on underscore prefixing.