[ThinLTO] Clean up stale alias import handling

Summary:
Remove some code that was no longer needed. The first FIXME is
stale since we long ago started using the index to drive importing,
rather than doing force importing based on linkage type. And
now with r309278, we no longer import any aliases.

Reviewers: dblaikie

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D37266

llvm-svn: 312019