Remove redundant ifdef in FontPlatformDataWin.cpp

Remove #if OS(WIN) ifdef in FontPlatformDataWin.cpp as that file is
only ever compiled on windows.

TBR=eseidel@chromium.org
BUG=249099

Review URL: https://codereview.chromium.org/180123003

git-svn-id: svn://svn.chromium.org/blink/trunk@167823 bbb929c8-8fbe-4397-9dbb-9b2b20218538