T.P.Writers.Shared: add new function removeLinks.

The function converts links to spans. It is used, for example, to avoid
nested links.

The HTML writer used to put the description of nested links into small
caps, but uses a simple *span* now.

test/command/11124.md

@@ -0,0 +1,10 @@
+The heading is "unlinked" before adding it to the TOC.
+
+```
+% pandoc --to=latex
+<http://example.com/> {-}
+=========================
+^D
+\section*{\texorpdfstring{\url{http://example.com/}}{http://example.com/}}\label{httpexample.com}
+\addcontentsline{toc}{section}{{http://example.com/}}
+```

test/command/8738.md

@@ -2,5 +2,5 @@
 % pandoc -f native -t mediawiki
 [Link ("",[],[]) [Link ("",[],[]) [Str "test"] ("https://bar.example.com",""),Str "Test"] ("https://foo.example.com","")]
 ^D
-[https://foo.example.com testTest]
+[https://foo.example.com <span>test</span>Test]
 ```