Fix clippy warnings (#1385)

## Issue Addressed

NA

## Proposed Changes

Fixes most clippy warnings and ignores the rest of them, see issue #1388.

lighthouse/environment/src/executor.rs

@@ -79,7 +79,7 @@ impl TaskExecutor {
     /// This function generates prometheus metrics on number of tasks and task duration.
     pub fn spawn_blocking<F>(&self, task: F, name: &'static str)
     where
-        F: FnOnce() -> () + Send + 'static,
+        F: FnOnce() + Send + 'static,
     {
         let exit = self.exit.clone();
         let log = self.log.clone();

lighthouse/src/main.rs

@@ -3,7 +3,6 @@ extern crate clap;
 
 use beacon_node::ProductionBeaconNode;
 use clap::{App, Arg, ArgMatches};
-use clap_utils;
 use env_logger::{Builder, Env};
 use environment::EnvironmentBuilder;
 use eth2_testnet_config::HARDCODED_TESTNET;
@@ -272,5 +271,6 @@ fn run<E: EthSpec>(
     drop(validator_client);
 
     // Shutdown the environment once all tasks have completed.
-    Ok(environment.shutdown_on_idle())
+    environment.shutdown_on_idle();
+    Ok(())
 }