Removed ENABLE_WTF_MULTIPLE_THREADS and related #ifdefs

Removed ENABLE_WTF_MULTIPLE_THREADS and related #ifdefs
https://bugs.webkit.org/show_bug.cgi?id=68423

As discussed on webkit-dev. All ports build with threads enabled in WTF now.

This may break WinCE and other ports that have not built and tested with
this configuration. I've filed bugs for port maintainers. It's time for
WebKit to move forward.

Reviewed by Mark Rowe.

* wtf/CryptographicallyRandomNumber.cpp:
(WTF::ARC4Stream::ARC4RandomNumberGenerator::randomNumber):
(WTF::ARC4Stream::ARC4RandomNumberGenerator::randomValues):
* wtf/FastMalloc.cpp:
* wtf/Platform.h:
* wtf/RandomNumber.cpp:
(WTF::randomNumber):
* wtf/RefCountedLeakCounter.cpp:
(WTF::RefCountedLeakCounter::increment):
(WTF::RefCountedLeakCounter::decrement):
* wtf/ThreadingPthreads.cpp:
(WTF::initializeThreading):
* wtf/ThreadingWin.cpp:
(WTF::initializeThreading):
* wtf/dtoa.cpp:
(WTF::pow5mult):
* wtf/gtk/ThreadingGtk.cpp:
(WTF::initializeThreading):
* wtf/qt/ThreadingQt.cpp:
(WTF::initializeThreading):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@95511 268f45cc-cd09-0410-ab3c-d52691b4dbfc