Remove WTF::Conditional.

It is defined in wtf/TypeTraits.h, but it is not used anywhere. Also,
the argument names are misleading (If / Then should actually be If /
Else, or IfTrue / IfFalse). Last, it is not tested in TypeTraits.cpp,
unlike the other templates, and it is never used in the codebase.

BUG=164933

Review URL: https://codereview.chromium.org/53293006

git-svn-id: svn://svn.chromium.org/blink/trunk@161348 bbb929c8-8fbe-4397-9dbb-9b2b20218538