Why we cannot blindly remove this?

I'm 99.99% sure we can do, but there could be old Chromes that version still uses 'translate_language_blacklist'. Or, can we ignore such old Chromes?

On second though, this pref was introduced at the M31 and this is old enough (I think the usage is almost 0). OK, I'll remove this pref blindly. Thanks.

https://codereview.chromium.org/1978203002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e74d2bc1307e28656cc611446defc02350c22608

commit e74d2bc1307e28656cc611446defc02350c22608
Author: hajimehoshi <hajimehoshi@chromium.org>
Date: Wed May 18 08:08:44 2016

Translate: Remove the legacy pref 'translate_language_blacklist'

'translate_language_blacklist' was replaced by
'translate_blocked_languages' at M31, which almost no one use. Let's
remove this legacy pref and its migration logic for code health.

BUG= 591937 
TEST=components_unittest --gtest_filter="TranslatePrefTest.*"

Review-Url: https://codereview.chromium.org/1978203002
Cr-Commit-Position: refs/heads/master@{#394357}

[modify] https://crrev.com/e74d2bc1307e28656cc611446defc02350c22608/chrome/browser/ui/webui/translate_internals/translate_internals_handler.cc
[modify] https://crrev.com/e74d2bc1307e28656cc611446defc02350c22608/components/translate/core/browser/translate_prefs.cc
[modify] https://crrev.com/e74d2bc1307e28656cc611446defc02350c22608/components/translate/core/browser/translate_prefs.h
[modify] https://crrev.com/e74d2bc1307e28656cc611446defc02350c22608/components/translate/core/browser/translate_prefs_unittest.cc