Remove redundant otherwise guard in inlineToAsciiDoc (#6146)

The `RawInline` case in `inlineToAsciiDoc` currenty looks like this:

```hs
inlineToAsciiDoc _ il@(RawInline f s)
  | f == "asciidoc" = return $ literal s
  | otherwise         = do
      report $ InlineNotRendered il
      return empty
  | otherwise       = return empty
```

Notice how there are there are two overlapping `otherwise` guards.
The second `otherwise` guard is completely unreachable, so this patch
removes it.

src/Text/Pandoc/Writers/AsciiDoc.hs

@@ -492,7 +492,6 @@ inlineToAsciiDoc _ il@(RawInline f s)
   | otherwise         = do
       report $ InlineNotRendered il
       return empty
-  | otherwise       = return empty
 inlineToAsciiDoc _ LineBreak = return $ " +" <> cr
 inlineToAsciiDoc _ Space = return space
 inlineToAsciiDoc opts SoftBreak =