Follow-up to r229740, which removed `DITemplate*::getContext()` after my
upgrade script revealed that scopes are always `nullptr` for template
parameters. This is the other shoe: drop `scope:` from
`MDTemplateParameter` and its two subclasses. (Note: a bitcode upgrade
would be pointless, since the hierarchy hasn't been moved into place.)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229791 91177308-0d34-0410-b5e6-96231b3b80d8
