Fix warnings and compile errors on nightly (#6886)

Nightly has added a bunch of compiler and linter warnings. There is also
two dependencies that fail compilation on latest nightly due to using
the old `stdsimd` feature name. This PR fixes them.

proxy/src/serverless/conn_pool.rs

@@ -667,7 +667,6 @@ impl<C: ClientInnerExt> Drop for Client<C> {
 
 #[cfg(test)]
 mod tests {
-    use env_logger;
     use std::{mem, sync::atomic::AtomicBool};
 
     use super::*;