Remove unnecessary PLATFORM() tests
https://bugs.webkit.org/show_bug.cgi?id=107360

Patch by Laszlo Gombos <l.gombos@samsung.com> on 2013-01-18
Reviewed by Eric Seidel.

PLATFORM(WIN), PLATFORM(CHROMIUM) and PLATFORM(QT) are mutually
exclusive. Remove redundant PLATFORM() tests based on this
invariant.

* wtf/Platform.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140238 268f45cc-cd09-0410-ab3c-d52691b4dbfc