Drop an unnecessary use of writev.

Drop an unnecessary use of writev.

It looks like the code this patch deletes is based on a misunderstanding of
what guarantees writev provides. In particular, writev with 1 iovec is
not "more atomic" than a write.

Testing on OS X shows that both write and writev from multiple processes
can be intermixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255837 91177308-0d34-0410-b5e6-96231b3b80d8