Don't build docs for removed libcollections.

Author: leoyvens

src/bootstrap/doc.rs

@@ -490,7 +490,7 @@ impl Step for Std {
         // for which docs must be built.
         if !build.config.compiler_docs {
             cargo.arg("--no-deps");
-            for krate in &["alloc", "collections", "core", "std", "std_unicode"] {
+            for krate in &["alloc", "core", "std", "std_unicode"] {
                 cargo.arg("-p").arg(krate);
                 // Create all crate output directories first to make sure rustdoc uses
                 // relative links.