EPUB writer: allow choice of math method for v3.

Previously we always used MathML for math in EPUB3, because
the spec includes MathML.  But this is not widely supported
by readers, so it seems better to allow users to choose their
math method as they can with EPUB2 or HTML.

Closes #8164.

NOTE!  Existing workflows that produce EPUBv3 documents including
math will be affected by this change.  You must add `--mathml` to
your command line if you want to continue producing MathML.

src/Text/Pandoc/Writers/EPUB.hs

@@ -491,10 +491,6 @@ pandocToEPUB version opts doc = do
       opts' = opts{ writerEmailObfuscation = NoObfuscation
                   , writerSectionDivs = True
                   , writerVariables = vars
-                  , writerHTMLMathMethod =
-                       if epub3
-                          then MathML
-                          else writerHTMLMathMethod opts
                   , writerWrapText = WrapAuto }
 
   -- cover page