Avoid displaying double borders for inline code snippets
Before we started using the 'code' tag for inline code snippets,
<code><syntaxhighlight enclose=none ...>...</syntaxhighlight></code>
was a common pattern. Continue supporting it in existing content.
Follow-up to 04293baad9e29d5b0283968b546c948f317338bd. My IRC comments
about this were seemingly forgotten, and the code I added in
5b7522a5fc4088792494c662cd06a794d9f77980 to handle this problem