Bug 1662838 part 2: Remove nsIContentViewer's unused pausePainting() and resumePainting() APIs. r=tnikkel

These APIs are entirely unused (aside from one usage in a test, which part 1 in
this patch series removed), so this patch shouldn't impact behavior at all.

Historical note: we briefly removed these APIs once before, in this commit:
https://hg.mozilla.org/mozilla-central/rev/c216ff19d690
...but we brought them back because we had a motivating use case at the time.
We don't have any such motivating use cases any more, though.  So, this patch
here is essentially a modernized version of that older commit.

Differential Revision: https://phabricator.services.mozilla.com/D108148

layout/base/nsDocumentViewer.cpp

@@ -2690,28 +2690,6 @@ nsDocumentViewer::SetHintCharset(const Encoding* aEncoding) {
   CallChildren(childFn);
 }
 
-NS_IMETHODIMP
-nsDocumentViewer::PausePainting() {
-  CallChildren([](nsDocumentViewer* aChild) { aChild->PausePainting(); });
-
-  if (PresShell* presShell = GetPresShell()) {
-    presShell->PausePainting();
-  }
-
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-nsDocumentViewer::ResumePainting() {
-  CallChildren([](nsDocumentViewer* aChild) { aChild->ResumePainting(); });
-
-  if (PresShell* presShell = GetPresShell()) {
-    presShell->ResumePainting();
-  }
-
-  return NS_OK;
-}
-
 nsresult nsDocumentViewer::GetContentSizeInternal(int32_t* aWidth,
                                                   int32_t* aHeight,
                                                   nscoord aMaxWidth,