docs: Remove documentation for legacy PGO options

Late last year r191835 removed a largely unmaintained legacy PGO
infrastructure, but some of the docs were missed. Since these docs are
for things that don't actually exist anymore, they should be removed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210165 91177308-0d34-0410-b5e6-96231b3b80d8