Remove support for the IOS_SIMULATOR_DEPLOYMENT_TARGET env var.

It turns out that this was never used. Instead we just use the
IPHONEOS_DEPLOYMENT_TARGET variable for both iOS devices and simulator.
rdar://problem/18596744

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219467 91177308-0d34-0410-b5e6-96231b3b80d8