Fix doxygen warnings about deprecated tags
Doxygen 1.8.10 warns that those tags are obsolete. Since we're not generating
XML anyway, it seems harmless to remove them even for earlier versions.
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index 6a00f47..e5c9cbb 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -1485,13 +1485,13 @@
# which can be used by a validating XML parser to check the
# syntax of the XML files.
-XML_SCHEMA =
+#XML_SCHEMA =
# The XML_DTD tag can be used to specify an XML DTD,
# which can be used by a validating XML parser to check the
# syntax of the XML files.
-XML_DTD =
+#XML_DTD =
# If the XML_PROGRAMLISTING tag is set to YES Doxygen will
# dump the program listings (including syntax highlighting