Remove krate.version; fix crate_version in JSON

Previously, `JsonRenderer::after_krate` called `krate.version.clone()`.
The problem was it did that after the version was already moved into the
cache, so it would always be None. The fix was to get the version from
the cache instead.

src/librustdoc/formats/cache.rs

@@ -144,7 +144,6 @@ impl Cache {
         dst: &Path,
     ) -> clean::Crate {
         // Crawl the crate to build various caches used for the output
-        self.crate_version = krate.version.take();
         debug!(?self.crate_version);
         self.traits = krate.external_traits.take();
         self.masked_crates = mem::take(&mut krate.masked_crates);