Revert "Adding DIImportedModules to DIScopes."

This reverts commit 342d92c7a0adeabc9ab00f3f0d88d739fe7da4c7.

Turns out we're going with a different schema design to represent
DW_TAG_imported_modules so we won't need this extra field.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178215 91177308-0d34-0410-b5e6-96231b3b80d8