Revert "Free created FileMap when uncompressing files"

This reverts commit 52b8235238fb4d8cf141020cc1ae33880929dc3f.

munmap() on a region that overlaps something else (e.g, the ZIP Central
Directory in this case) unmaps those other regions as well making the
ZipFileRO class unhappy. Revert this for now until we can have FileMap
deal with this craziness.