rustdoc: Remove unnecessary optional

Previously, the HTML output format was represented by both
`Some(OutputFormat::Html)` and `None` so there's no need to have an
optional. Instead, `OutputFormat::Html` is explicitly the default and we
no longer have a "tri-state enum".

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -132,7 +132,7 @@ impl<'a, 'b> CoverageCalculator<'a, 'b> {
 
     fn print_results(&self) {
         let output_format = self.ctx.renderinfo.borrow().output_format;
-        if output_format.map(|o| o.is_json()).unwrap_or_else(|| false) {
+        if output_format.is_json() {
             println!("{}", self.to_json());
             return;
         }