[mips] Drop the claim that ubsan works since r243384 and r244646 are not yet merged.

[mips] Drop the claim that ubsan works since r243384 and r244646 are not yet merged.

The timezone difference between myself, the code-owner, and release manager means
it's sensible to update the release notes on the assumption that they won't be
merged. If we do merge them, then we can revert this release notes change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_37@245037 91177308-0d34-0410-b5e6-96231b3b80d8