[debuginfo] Remove temporary FIXME.

Summary:
Now that r320495, "[debuginfo-tests] Support moving
debuginfo-tests to llvm/projects," has landed, remove temporary FIXME
that supported the old mechanism.

Reviewers: zturner, aprantl

Reviewed By: aprantl

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D41259

llvm-svn: 320751