Revert "Set test flag when rustdoc is running with --test option"

This reverts commit 8ed2292dbe.

It caused doctests in this repository to no longer be tested including all of the core crate.

src/librustdoc/config.rs

@@ -351,9 +351,6 @@ impl Options {
                             .unwrap_or_else(|| PathBuf::from("doc"));
         let mut cfgs = matches.opt_strs("cfg");
         cfgs.push("rustdoc".to_string());
-        if should_test {
-            cfgs.push("test".to_string());
-        }
 
         let extension_css = matches.opt_str("e").map(|s| PathBuf::from(&s));