Remove mmap_unaligned.

Remove mmap_unaligned, which was used to heuristically decide whether to
optimistically call mmap() in such a way that could reduce the total
number of system calls.  If I remember correctly, the intention of
mmap_unaligned was to avoid always executing the slow path in the
presence of ASLR.  However, that reasoning seems to have been based on a
flawed understanding of how ASLR actually works.  Although ASLR
apparently causes mmap() to ignore address requests, it does not cause
total placement randomness, so there is a reasonable expectation that
iterative mmap() calls will start returning chunk-aligned mappings once
the first chunk has been properly aligned.