Get rid of unused function PlainTextRangeTest::InsertHTMLElement()

This patch gets rid of unused function |PlainTextRangeTest::InsertHTMLElement()|
for improving code health.

Change-Id: I6f246b6a8f944acc3bfba119d08ad965edb5dd28
Reviewed-on: https://chromium-review.googlesource.com/c/1351320Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612521}

third_party/blink/renderer/core/editing/plain_text_range_test.cc

@@ -9,10 +9,7 @@
 
 namespace blink {
 
-class PlainTextRangeTest : public EditingTestBase {
- protected:
-  Element* InsertHTMLElement(const char* element_code, const char* element_id);
-};
+class PlainTextRangeTest : public EditingTestBase {};
 
 TEST_F(PlainTextRangeTest, RangeContainingTableCellBoundary) {
   SetBodyInnerHTML(