[scudo] Remove comment about security of the 32-bit allocator

Summary:
The 32-bit allocator is now on par with the 64-bit in terms of security (chunks
randomization is done, batches separation is done).

Unless objection, the comment can go away.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39303

llvm-svn: 316620