[MC][PGO][PGSO] Cleanup unused MBFI in AsmPrinter

Summary:
Machine Block Frequency Info (MBFI) is being computed but unused in AsmPrinter.

MBFI computation was introduced with PGO change D71149 and then its use was
removed in D71106. No need to keep computing it.

Reviewers: MaskRay, jyknight, skan, yamauchi, davidxl, efriedma, huihuiz

Reviewed By: MaskRay, skan, yamauchi

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78526