[PM] Remove now-dead extern template and explicit instantiation

[PM] Remove now-dead extern template and explicit instantiation
declarations.

We're using a custom class here instead of the helper template, these
bits just didn't get deleted when the other bits did get deleted. This
was found by a really nice MSVC warning about explicitly instantiating
a template where some member functions aren't defined and thus can't be
instantiatied.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290327 91177308-0d34-0410-b5e6-96231b3b80d8