Delete unneeded platform-specific expectations.

They are identical to the generic expectations. Found with:

for f in `find . -name '*_expected_*.png'`; do
  g=$(echo $f | sed 's/expected_.../expected/')
  ../../out/Debug/pdfium_diff $f $g > /dev/null && echo $f
done

Change-Id: Ia597810b3ebb854a915ce9ca1910aa3e0b28f363
Reviewed-on: https://pdfium-review.googlesource.com/38930
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>