deps: remove 'num_cpus'

Now that std:🧵:available_parallelism is a thing, we no longer
need num_cpus.

crates/core/main.rs

@@ -55,7 +55,7 @@ fn main() {
 fn try_main(args: Args) -> Result<()> {
     use args::Command::*;
 
-    let matched = match args.command()? {
+    let matched = match args.command() {
         Search => search(&args),
         SearchParallel => search_parallel(&args),
         SearchNever => Ok(false),