Revert "sanitizer_common: optimize memory drain"

This reverts commit 07266952.

This causes the following build failure with gcc 10.3.0:

/home/nikic/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_allocator_primary64.h:114:31: error: declaration of ‘typedef class __sanitizer::MemoryMapper<__sanitizer::SizeClassAllocator64<Params> > __sanitizer::SizeClassAllocator64<Params>::MemoryMapper’ changes meaning of ‘MemoryMapper’ [-fpermissive]
  114 |   typedef MemoryMapper<ThisT> MemoryMapper;