Only load sphinx_markdown_builder extension if it's used

As is, building man pages or HTML docs while sphinx_markdown_builder
is installed, will result in unrelated warnings.  Remove those by
removing it from the extensions config.  Markdown building (only used
for changelog) seems to work without this just fine.

(cherry picked from commit 7b495497d7)

doc_src/conf.py

@@ -16,10 +16,6 @@ from docutils import nodes
 
 try:
     import sphinx_markdown_builder
-
-    extensions = [
-        "sphinx_markdown_builder",
-    ]
 except ImportError:
     pass