sanitizer_common: remove debugging logic from the internal allocator

The internal allocator adds 8-byte header for debugging purposes.
The problem with it is that it's not possible to allocate nicely-sized
objects without a significant overhead. For example, if we allocate
512-byte objects, that will be rounded up to 768 or something.
This logic migrated from tsan where it was added during initial development,
I don't remember that it ever caught anything (we don't do bugs!).
Remove it so that it's possible to allocate nicely-sized objects
without overheads.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D105777