IR: Drop the scope in DI template parameters

The scope/context is always the compile unit, which we replace with
`nullptr` anyway (via `getNonCompileUnitScope()`).  Drop it explicitly.

I noticed this field was always null while writing testcase upgrade
scripts to transition to the new hierarchy.  Seems wasteful to
transition it over if it's already out-of-use.

llvm-svn: 229740