chore(proxy): remove postgres config parser and md5 support (#9990)

Keeping the `mock` postgres cplane adaptor using "stock" tokio-postgres
allows us to remove a lot of dead weight from our actual postgres
connection logic.

libs/proxy/postgres-protocol2/src/authentication/mod.rs

@@ -1,37 +1,2 @@
 //! Authentication protocol support.
-use md5::{Digest, Md5};
-
 pub mod sasl;
-
-/// Hashes authentication information in a way suitable for use in response
-/// to an `AuthenticationMd5Password` message.
-///
-/// The resulting string should be sent back to the database in a
-/// `PasswordMessage` message.
-#[inline]
-pub fn md5_hash(username: &[u8], password: &[u8], salt: [u8; 4]) -> String {
-    let mut md5 = Md5::new();
-    md5.update(password);
-    md5.update(username);
-    let output = md5.finalize_reset();
-    md5.update(format!("{:x}", output));
-    md5.update(salt);
-    format!("md5{:x}", md5.finalize())
-}
-
-#[cfg(test)]
-mod test {
-    use super::*;
-
-    #[test]
-    fn md5() {
-        let username = b"md5_user";
-        let password = b"password";
-        let salt = [0x2a, 0x3d, 0x8f, 0xe0];
-
-        assert_eq!(
-            md5_hash(username, password, salt),
-            "md562af4dd09bbb41884907a838a3233294"
-        );
-    }
-}