[turbofan] Remove unused typedef VirtualRegisterSet.
[turbofan] Remove unused typedef VirtualRegisterSet.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/1179913003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29010}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh