Reapply "ox-html: Use <img> tags for svgs, not <object>"

This reverts commit 302bc6393a.

It turns out that <object ...> tags with data pointing outside the
website are not rendered by the modern
qtwebengine (linked svgs in https://orgmode.org/tools.html are not
rendered). Even though Firefox does render the same page, it no longer
appears safe to use <object ...> for svg.  So, we have to use <img...>
with its limitations.

lisp/ox-html.el

@@ -1772,43 +1772,18 @@ SOURCE is a string specifying the location of the image.
 ATTRIBUTES is a plist, as returned by
 `org-export-read-attribute'.  INFO is a plist used as
 a communication channel."
-  (if (string= "svg" (file-name-extension source))
-      (org-html--svg-image source attributes info)
-    (org-html-close-tag
-     "img"
-     (org-html--make-attribute-string
-      (org-combine-plists
-       (list :src source
-	     :alt (if (string-match-p
-		       (concat "^" org-preview-latex-image-directory) source)
-		      (org-html-encode-plain-text
-		       (org-find-text-property-in-string 'org-latex-src source))
-		    (file-name-nondirectory source)))
-       attributes))
-     info)))
-
-(defun org-html--svg-image (source attributes info)
-  "Return \"object\" embedding svg file SOURCE with given ATTRIBUTES.
-INFO is a plist used as a communication channel.
-
-The special attribute \"fallback\" can be used to specify a
-fallback image file to use if the object embedding is not
-supported.  CSS class \"org-svg\" is assigned as the class of the
-object unless a different class is specified with an attribute."
-  (let ((fallback (plist-get attributes :fallback))
-	(attrs (org-html--make-attribute-string
-		(org-combine-plists
-                 ;; Remove fallback attribute, which is not meant to
-                 ;; appear directly in the attributes string, and
-                 ;; provide a default class if none is set.
-                 '(:class "org-svg") attributes '(:fallback nil)))))
-    (format "<object type=\"image/svg+xml\" data=\"%s\" %s>\n%s</object>"
-	    source
-	    attrs
-	    (if fallback
-		(org-html-close-tag
-		 "img" (format "src=\"%s\" %s" fallback attrs) info)
-	      "Sorry, your browser does not support SVG."))))
+  (org-html-close-tag
+   "img"
+   (org-html--make-attribute-string
+    (org-combine-plists
+     (list :src source
+           :alt (if (string-match-p
+                     (concat "^" org-preview-latex-image-directory) source)
+                    (org-html-encode-plain-text
+                     (org-find-text-property-in-string 'org-latex-src source))
+                  (file-name-nondirectory source)))
+     attributes))
+   info))
 
 (defun org-html--textarea-block (element)
   "Transcode ELEMENT into a textarea block.