css: remove font size adjustment for <code> elements

There's a difference between how Jekyll generates inline code
blocks from what we had in the banshee website: the former now
generates <pre><code>foo</code></pre> blocks instead of just
<pre>foo</pre>. This means that those elements were now
rendered with a much smaller font than before, because there
was a 0.80em font size adjustment for <code> tags.

There doesn't seem to be a good reason for such a small size
for <code> blocks though, so we can simply remove this font size
adjustment to make the website look better again, like this
example:

https://web.archive.org/web/20140208064930/http://banshee.fm/contribute/file-bugs/