Remove layout tests that run execCommand on non-HTML documents

LayoutTests/svg/custom/bug78838.html

Index: LayoutTests/svg/custom/bug78838.html
diff --git a/LayoutTests/svg/custom/bug78838.html b/LayoutTests/svg/custom/bug78838.html
deleted file mode 100644
index f39f14d2afe0703cc08444dcc14c2050e16e0cf9..0000000000000000000000000000000000000000
--- a/LayoutTests/svg/custom/bug78838.html
+++ /dev/null
@@ -1,19 +0,0 @@
-<!-- This test passes if there is no crash. -->
-<script>
-function go() {
-
-    if (window.testRunner)
-        testRunner.dumpAsText();
-
-    q = document.getElementById('root').contentDocument;
-    q.firstChild.setAttribute('style', 'content:counter(item)');
-
-    document.designMode = 'on';
-    q.execCommand('selectAll');
-    q.execCommand('insertImage');
-    q.getElementById('x').appendChild( q.firstChild.cloneNode(1) );
-    q.execCommand('undo');
-}
-</script>
-<object data="resources/bug78838.svg" id="root" onload="go()"/></object>
-<p>PASSED</p>