Remove unused FontPlatformDataHarfBuzz::emSizeInFontUnits

Remove the unused m_emSizeInFontUnits field and emSizeInFontUnits
method from FontPlatformDataHarfBuzz.

BUG=355806
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/210573002

git-svn-id: svn://svn.chromium.org/blink/trunk@169955 bbb929c8-8fbe-4397-9dbb-9b2b20218538