Remove ReentrantMutex

This drops the parking_lot dependency; the ReentrantMutex type appeared
to be unused (at least, no compilation failures occurred).

This is technically a possible change in behavior of its users, as
lock() would wait on other threads releasing their guards, but since we
didn't actually remove any threading or such in this code, it appears
that we never used that behavior (the behavior change is only noticeable
if the type previously was used in two threads, in a single thread
ReentrantMutex is useless).

src/librustdoc/html/render.rs

@@ -659,7 +659,7 @@ pub fn run(mut krate: clean::Crate,
         crate_version: krate.version.take(),
         orphan_impl_items: Vec::new(),
         orphan_trait_impls: Vec::new(),
-        traits: krate.external_traits.lock().replace(Default::default()),
+        traits: krate.external_traits.replace(Default::default()),
         deref_trait_did,
         deref_mut_trait_did,
         owned_box_did,