[hwasan] Remove system allocator fallback.

Summary:
This has been an experiment with late malloc interposition, made
possible by a non-standard feature of the Android dynamic loader.

Reviewers: pcc, mmalcomson

Subscribers: srhines, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69199

llvm-svn: 375296