Remove unused is_prerelease template variable

Author: GuillaumeGomez

templates/crate/build_details.html

@@ -11,7 +11,6 @@
 
 {%- block topbar -%}
   {%- set inner_path = metadata.target_name_url() -%}
-  {%- set is_prerelease = false -%}
   {%- set has_crate = false %}
   {%- include "rustdoc/topbar.html" -%}
 {%- endblock topbar -%}