[NFC] Remove inaccurate comment

Summary:
r271558 moved getManagedStaticMutex's mutex from a function-local
static to using call_once, but left a comment added in r211424. That comment is
now erroneous, remove it.

Reviewers: zturner, chandlerc

Subscribers: aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D46784

llvm-svn: 332175