[RemoveLegacy] Delete some legacy code paths in NGBoxFragmentPainter.

Remove FragmentRequiresLegacyFallback(), and behave as if it always
returned false.

One change was necessary: NGBoxFragmentPainter::PaintBoxItem() now needs
to call PaintFragment() instead of setting up an NGBoxFragmentPainter on
its own, so that we enter the LayoutObject paint methods when necessary
(instead of using NGBoxFragmentPainter). Replaced content still requires
the LayoutObject painters.

This also eliminates use of DisplayItemCacheSkipper. We used to skip the
display item cache when FragmentRequiresLegacyFallback() was true, but
this was actually only necessary if we were to enter legacy containers
during painting. That is no longer possible. The only way we can get
legacy containers these days is if LayoutNGPrinting is disabled, but in
that case the entire document will use legacy layout, and we'll never
enter NGBoxFragmentPainter in the first place.

Bug: 1229581
Change-Id: I3a7655f3022e7805ff10752d026a3c1073878565
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4311044
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1113547}