[ValueMapper] NFC: Remove dead code to pause metadata mapping

Summary:
This functionality was added when Mapper::mapMetadata was recursive.  It
is no longer needed after r265456, which switched it to be iterative.

Reviewers: dexonsmith, srhines

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66860

llvm-svn: 370236