Revert: Run the FastMalloc scavenger thread on iOS as well.
<https://webkit.org/b/134927>
<rdar://problem/17485079>

This appears to be the cause of some elusive crashes, and since I don't
have a way to reproduce them, let's just go back to not running the
scavenger thread on iOS.

Reviewed by Antti Koivisto.

* wtf/FastMalloc.cpp:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171107 268f45cc-cd09-0410-ab3c-d52691b4dbfc