Remove stabilized library features from book.

Author: Mark-Simulacrum

src/doc/unstable-book/src/SUMMARY.md

@@ -109,7 +109,6 @@
     - [collection_placement](library-features/collection-placement.md)
     - [collections_range](library-features/collections-range.md)
     - [collections](library-features/collections.md)
-    - [command_envs](library-features/command-envs.md)
     - [compiler_builtins_lib](library-features/compiler-builtins-lib.md)
     - [compiler_fences](library-features/compiler-fences.md)
     - [concat_idents_macro](library-features/concat-idents-macro.md)
@@ -169,7 +168,6 @@
     - [once_poison](library-features/once-poison.md)
     - [oom](library-features/oom.md)
     - [option_entry](library-features/option-entry.md)
-    - [osstring_shrink_to_fit](library-features/osstring-shrink-to-fit.md)
     - [panic_abort](library-features/panic-abort.md)
     - [panic_unwind](library-features/panic-unwind.md)
     - [pattern](library-features/pattern.md)
@@ -181,7 +179,6 @@
     - [rand](library-features/rand.md)
     - [range_contains](library-features/range-contains.md)
     - [raw](library-features/raw.md)
-    - [reverse_cmp_key](library-features/reverse-cmp-key.md)
     - [rt](library-features/rt.md)
     - [rustc_private](library-features/rustc-private.md)
     - [sanitizer_runtime_lib](library-features/sanitizer-runtime-lib.md)
@@ -203,7 +200,6 @@
     - [str_box_extras](library-features/str-box-extras.md)
     - [str_mut_extras](library-features/str-mut-extras.md)
     - [test](library-features/test.md)
-    - [thread_id](library-features/thread-id.md)
     - [thread_local_internals](library-features/thread-local-internals.md)
     - [thread_local_state](library-features/thread-local-state.md)
     - [toowned_clone_into](library-features/toowned-clone-into.md)