rustdoc: remove unused 16x16 favicon

According to <https://caniuse.com/?search=svg%20favicon>,
SVG favicons are supported in everything but Safari.

When I actually try it in Safari, it's downloading all
three favicons, and nothing looks different when I disable
the 16x16 one.

<https://dev.to/masakudamatsu/favicon-nightmare-how-to-maintain-sanity-3al7>,
which is linked from caniuse above, recommends an ico.
However, the reason they recommend it is the apps that
only support /favicon.ico exactly, and rustdoc can't assume
it will be installed to the site root, so it's unfortunately
up to the webmaster to make sure it's set up.

src/librustdoc/html/static_files.rs

@@ -106,7 +106,6 @@ static_files! {
     license_mit => "static/LICENSE-MIT.txt",
     rust_logo_svg => "static/images/rust-logo.svg",
     rust_favicon_svg => "static/images/favicon.svg",
-    rust_favicon_png_16 => "static/images/favicon-16x16.png",
     rust_favicon_png_32 => "static/images/favicon-32x32.png",
     fira_sans_regular => "static/fonts/FiraSans-Regular.woff2",
     fira_sans_medium => "static/fonts/FiraSans-Medium.woff2",

src/librustdoc/html/templates/page.html

@@ -58,8 +58,6 @@
     {% if !layout.favicon.is_empty() %}
         <link rel="icon" href="{{layout.favicon}}"> {# #}
     {% else %}
-        <link rel="alternate icon" type="image/png" {#+ #}
-            href="{{static_root_path|safe}}{{files.rust_favicon_png_16}}"> {# #}
         <link rel="alternate icon" type="image/png" {#+ #}
             href="{{static_root_path|safe}}{{files.rust_favicon_png_32}}"> {# #}
         <link rel="icon" type="image/svg+xml" {#+ #}