HTML validation: Remove invalid <button href> attribute.

For .start-button, Bootstrap carousel already supports <button
data-target> as a valid alternative to <button href>.  For
.call-to-action, the margin is decreased to exactly offset the lack of
margin collapsing with display: inline-block.  There should be no
visual change.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

tools/documentation_crawler/documentation_crawler/spiders/common/spiders.py

@@ -33,8 +33,6 @@ VNU_IGNORE = re.compile(r'|'.join([
     r'No “p” element in scope but a “p” end tag seen\.',
     r'Element “div” not allowed as child of element “ul” in this context\. '
     + r'\(Suppressing further errors from this subtree\.\)',
-    r'The element “button” must not appear as a descendant of the “a” element\.',
-    r'Attribute “href” not allowed on element “button” at this point\.',
 
     # Warnings that are probably less important.
     r'The “type” attribute is unnecessary for JavaScript resources\.',