[JumpThread] No need to erase BB from LoopHeaders. NFC.

Summary: No need to try to ease BB from LoopHeaders as we already know that BB is not in LoopHeaders.

Reviewers: hsung, majnemer, mcrosier, haicheng, rengolin

Reviewed By: rengolin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29232

llvm-svn: 293802