IR: Drop scope from MDTemplateParameter

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.)

llvm-svn: 229791