rustdoc: clean up sidebar.html block class

This line is longer than 100 characters, but apparently,
[tidy's list of checked extensions] doesn't include html,
so the line length doesn't matter.

[tidy's list of checked extensions]: 31be8cc411/src/tools/tidy/src/style.rs (L245)

src/librustdoc/html/templates/sidebar.html

@@ -18,11 +18,7 @@
                         <h3><a href="#{{block.heading.href|safe}}">{{block.heading.name}}</a></h3>
                     {% endif %}
                     {% if !block.links.is_empty() %}
-                        <ul {%+ if block.class.is_empty() %}
-                                class="block"
-                            {% else %}
-                                class="block {{+ block.class}}"
-                            {% endif %}>
+                        <ul class="block{% if !block.class.is_empty() +%} {{+block.class}}{% endif %}">
                             {% for link in block.links %}
                                 <li><a href="#{{link.href|safe}}">{{link.name}}</a></li>
                             {% endfor %}