Revert "[llvm-profdata] Speed up merging by using a thread pool"

This reverts commit r275921. It broke the ppc64be bot:

  http://lab.llvm.org:8011/builders/clang-ppc64be-linux-multistage/builds/3537

I'm not sure why it broke, but based on the output, it looks like an
off-by-one (one profile left un-merged).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275937 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed