Remove unused text encoding quirks.

1) Remove more Backslash-as-JPY hacks (some encodings interpret backslash as yen
sign).

The code was added in https://bugs.webkit.org/show_bug.cgi?id=49714 (r73566).

The test used to be
editing/selection/find-yensign-and-backslash-with-japanese-fonts-expected.html

The feature was removed by https://codereview.chromium.org/27030014 (r159589),
but the removal was incomplete. This CL removes the leftover code and test
expectation files (the corresponding tests were removed by r159589).

2) Remove isJapaneseEncoding from TextEncodingRegistry.cpp; it was dead
code. There used to be bool TextEncoding::isJapanese() (see r16245) which was
used by WebCore/loader/Decoder.cpp... but TextEncoding::isJapanese no longer
exists, and isJapaneseEncoding appears not to be used any more.

BUG=
R=abarth@chromium.org, jshin@chromium.org

Review URL: https://codereview.chromium.org/335803002

git-svn-id: svn://svn.chromium.org/blink/trunk@176407 bbb929c8-8fbe-4397-9dbb-9b2b20218538