docs: Update HowToSetUpLLVMStyleRTTI.
docs: Update HowToSetUpLLVMStyleRTTI.

Recent changes to isa<>/dyn_cast<> have made unnecessary those classof()
of the form:

    static bool classof(const Foo *) { return true; }

Accordingly, remove mention of such classof() from the documentation.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@165766 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,dslab-epfl/asap,chubbymaggie/asap,apple/swift-llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,chubbymaggie/asap,llvm-mirror/llvm,dslab-epfl/asap,llvm-mirror/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,dslab-epfl/asap,dslab-epfl/asap,apple/swift-llvm,chubbymaggie/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,chubbymaggie/asap,llvm-mirror/llvm,chubbymaggie/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,chubbymaggie/asap