Remove the unused return value in ASTImporter::Imported [NFC]

Summary:
`ASTImporter::Imported` currently returns a Decl, but that return value is not used by the ASTImporter (or anywhere else)
nor is it documented.

Reviewers: balazske, martong, a.sidorin, shafik

Reviewed By: balazske, martong

Subscribers: rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59595

llvm-svn: 356592